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Volvox : Characteristics, Structure, and Reproduction

Volvox is a common freshwater free-floating chlorophytic green alga that belongs to Volvocaceae family under order Volvocales of division Chlorophyta. They occur in temporary and permanent freshwater tanks, ponds, pools, ditches, etc. There are some 20 freshwater species of Volvox which prefer to live in colonies with up to 60,000 cells by making a gelatinous wall. These colonies have an ovoid or spherical hollow shape which may be larger than a pinhead size. Dutch microscopist Antonie van Leeuwenhoek first reported the Volox colonies in 1700.

The mature Volvox colony contains two separate cell types namely germ cells of the smaller number and numerous flagellated somatic cells. In this case, adult somatic cells have a single layer that contains two flagella which allow the organism to swim in a coordinated fashion in water. The cells have distinct anterior and posterior poles. The anterior pole possesses photosensitive eyespots that make it possible for the colony to move towards the light. The Volvox colonies are asexual which produce daughter colonies within the parent colony. After maturing, the daughter colony comes out from the parent colony.

Each Volvox species are able to make its own food through photosynthesis due to the presence of chlorophyll in its body. During spring, the surface of the water in which Volox occurs looks green. During early summer, the Volox abruptly disappears and it remains in resting zygote condition. Volvox colony appears in the rainy season.

Characteristics Features of Volvox

  • The Volvox cell is single, ovoid or spherical in shape which contains two flagella and it appears like a minute floating ball of a pinhead size.
  • The base of the flagella bears single cup-shaped chloroplasts.
  • Each individual cell is attached to each other with cytoplasmic strands.
  • Each individual cell possesses a red eyespot on its surface.
  • Anterior cells of the particular colony of Volvox possess phototactic abilities while the posterior cells perform reproduction.
  • The size of the Volvox colony ranges from 100-6000 µm.
  • We cannot see the Volvox species with the naked eye due to their microscopic size but few colonies are easily visible because of their big size with 1 mm in diameter.
  • Volvox prefers to live in nutrient-rich water bodies such as lakes, pools, canals, ditches, etc.
  • Each cell of the Volvox colony produces mucilage which makes the colony distinct or inconspicuous.
  • They show the flagellar movement. In this case, the flagella of all the cells of the colony perform simultaneous action by which the entire colony rolls over the surface of the water. Besides these, the eyespot controls the movement of the flagella as they are photoreceptive organs.
  • They reproduce both asexually and sexually.
  • They can be dioecious or monoecious. In this case, the male colony produces lots of sperm packets while the female colony releases oogamete or ovum.

Systematic Position of Volvox

  • Division: Chlorophyta
  • Class: Chlorophyceae
  • Order: Volvocales
  • Family: Volvocaceae
  • Genus:Volvox
  • Species:Volvox globator, V. minor, V. aureus, V. africanus, V. prolificus

Structure of the Vegetative Body

Volvox occurs in the colony because it is a coenobial form (hollow ball) like a structure. In the young colony, the vegetative cells are similar in size and green in color. Each motile colony (coenobium) is free-swimming and appears as small pinhead like spherical to ovoid shape with hollow mucilaginous mass which consists of numerous small pear-shaped cells arranged in a single layer joined with one another by delicate strands of cytoplasm within the periphery of the gelatinous colonial matrix. The total number of cells in the colony varies from about 500 (Volox aureus) to about 2000 or more (Volvox globate). Each colony develops the following three types of cells:

image of Volvox colony

Volvox  colony

  • Vegetative Cells: These cells are flagellated and vegetative in nature and are unable of giving rise to new colonies. They are involved in locomotion and capable of food production.
  • Asexual Reproductive Cells: These cells are larger in size than the vegetative cell and produce zoospores.
  • Sexual Reproductive cells: These cells are also larger in size producing sperms and eggs.

Cell Structure of Volvox

The cell of the coenobium varies based on species and is mostly ovoid-shaped. Each cell measured about 16.25 µm in length. The cell wall encloses a mass of protoplast. The cell wall is thin and firm in nature composed of cellulose. Protoplast contains a basal cup-shaped chloroplast with several pyrenoids (Volvox aureus) or plate-shaped with a single pyrenoid (Volovox globator), a central nucleus, reddish-brown eyespot surrounded by a plasma membrane. The central cytoplasm possesses mitochondria, endoplasmic reticulum, ribosome, dictyosomes, etc. At the apical portion of the cell, two equal length whiplash types of flagella arise from the two basal granules, i.e. the blepharoplast. At the base of the flagella, 2-3 contractile vacuoles are present. They act as excretory organs.

Locomotion of Volvox

The Volvox coenobium (colony) is motile and movement is brought by the simultaneous action of the flagella of all the cells of the colony. The entire colony rolls over the surface of the water. Hence they are called ‘rolling algae’. The eyespot controls the movement of the flagella as they are photoreceptive organs.

Reproduction in Volvox

Volvox species are either dioecious or monoecious. In this case, the male colony produces lots of sperm packets while the female colony releases oogamete or ovum. They are facultatively sexual but can reproduce both asexually and sexually. The environmental factors and sex-inducing pheromone trigger Volvox reproduction.

As the colony grows older, several cells in the posterior region lose their flagella and increase ten or more times; these enlarged cells are reproductive cells and may be asexual or sexual. The vegetative or somatic cells are unable to take part in reproduction. Sexual reproduction occurs through the formation of sperms and egg cells. The process of sperm and egg production is known as spermatogenesis and oogenesis, respectively. Asexual reproduction takes place at the beginning of the growing season whereas sexual reproduction occurs at the end of the growing season.

Asexual Reproduction in Volvox

Asexual reproduction takes place during summer in a rapid manner under favorable conditions. In a young colony known, as coenobium, all the cells are the same but later, a few cells of the posterior half of the Volvox colony increase in size by storing up the food. These cells become enlarged in size and form asexual reproductive cells, called gonidia or parthenogonidia. The number of gonidia varies from 2-50 in each coenobium. They drop their flagella, become rounded in outline, contain dense cytoplasm and lie within the globose mucilaginous sac which projects towards the inside of the colony.

Each gonidium divides repeatedly and produces a spherical group of daughter cells. In this case, all cells are held together to form a new daughter colony. The divisions of the gonidial protoplast occurring in the formation of a daughter colony are always longitudinal and all cells of each cell generation divide at the same time. Continue longitudinal divisions of daughter cells occur simultaneously and produce several cell generations. 

In the second generation, four cells are arranged quadrately while in the third cell generation, the 8 cells are crucially arranged, to form a curved plate, known as the plakea stage. Plakea takes the shape of a hollow sphere at the end of the 16-celled stage. Next, a pore called the phialopore is formed at the anterior pole of the daughter colony, when the cell division stops. 

The young daughter colony turns itself out by inverting through the phialopore. After this, the cells develop flagella and the daughter colony escapes by moving through a pore-like opening at the free face of the sac. Finally, the daughter colony comes out due to the rupture or decay of the mother colony or coenobium.

image of Asexual reproduction of volvox

Image showing stages of asexual reproduction of Volvox

Sexual Reproduction in Volvox

Sexual reproduction of Volovx is of oogamous type. The coenobium may be homothallic or heterothallic based on species. The posterior half of the coenobium forms some specialized enlarged cells or gametangia which may be either the female sex organs (oogonia) or the male sex organ (antheridia). They are produced fewer in number. 

During the development of gametangia (oogonia or antheridia), the cell becomes rounded and enlarged and cast off flagella but they remain linked with other cells through fine protoplasmic threads. In this case, the male sex organ or gametangium is called antheridium and the female sex organ or gametangium is known as oogonium. 

In the monoecious species, such as Volvox globator, antheridia and oogonia are formed on the same coenobium but in the dioecious species such as in Volvox aureus, antheridia and oogonia are formed on different coenobium. In monoecious species, antheridia develop first and the fertilization occurs between the antherozoid and ovum of other plants.

Antheridium

Antheridium also possesses an enlarged structure similar to gonidia. The protoplast of an antheridium divides repeatedly to form 16, 32, 64, 128 or more small, spindle-shaped, yellowish, biflagellate antherozoids. Each antherozoid contains a single nucleus and a small pale green or yellow-green chloroplast.

image of Development of Antherozoids

Oogonium

Oogonium is a unicellular, enlarged, semi flask-shaped cell, with a gelatinous sheath-like wall. The protoplast of each oogonium forms a larger uni-nucleate spherical oosphere or egg with a beak-like protrusion towards one side. Antherozoid enters into the oogonium through this end. 

The oosphere possesses a parietal chloroplast, pyrenoids, and a centrally placed large nucleus. Oogonium absorbs reserve substances from the neighboring cells through the protoplasmic strands.

image of Oogonium and Fertilization

Fertilization

During the fertilization, antherozoids after liberation from the antheridium swim about as a group and remain intact until they reach the egg. Only one antherozoid fuses with the egg resulting in the formation of a zygote or oospore. After fertilization, the zygote develops a thick wall around it. 

The protoplast of the zygote becomes orange-red in color. The zygote comes out of the parent coenobium by the disintegration of the gelatinous matrix of the coenobium and sinks to the bottom of the water and undergoes a period of rest.

Germination of Zygote

The zygote reserves enough food materials with other inclusions. The two outer layers of the zygote split and gelatinize. The outer layer is known as exospore which may be smooth in Volvox globator or spiny in Volvox speematospaera. The middle layer is known as mesopore while the inner layer is endospore. After releasing from the coenobium by disintegrating the gelatinous matrix, the zygote settles down at the bottom of the water body and may remain intact for several years.

Under favorable conditions, the inner wall layer extrudes out in the form of a vesicle and surrounds the protoplast of the zygote. Diploid zygote nucleus divides meiotically into four haploid nuclei; of these, 3 degenerates and the remaining one nucleus survive with cytoplasmic contents escapes from the vesicle. At this stage, it is known as a swarmer who swims freely and forms a zoospore and develops into a new coenobium (colony).

Each coenobium also contains a smaller number of cells which perform asexual reproduction for the next several generations. In the case of Volvox rouseletti and Volvox minor, the zygote`s protoplasm is changed into a single zoospore and it divides again to form a new coenobium.

image of Germination of zygote of Volvox

Concluding Remarks

There are about 20 species of Volvox worldwide. They are an important part of the aquatic ecosystem as primary producers. They can produce oxygen during photosynthesis which is needed in significant quantities by many aquatic life forms. They also act as a part of the food chain which makes them an important component of the food items of many aquatic organisms such as fish.

Coelom Vs Pseudocoelom: Definition, Types, Differences, Functions and Examples

The liquid filled cavity between alimentary canal and body wall of multicellular triploblastic animals is known as body cavity. Generally, the body cavity that is lined by mesodermal peritoneal membrane is known as coelom. According to Hyman (1955), coelom is the hollow space between alimentary canal and body wall which is lined by mesodermal peritoneal tissues. Coelom gives the space for most of the visceral organs. Between two embryonic layers of mesoderm, coelom originates as a secondary body cavity.  Haeckel first proposed the term coelom in 1972.

Coelom is also known as the perivisceral cavity that has fluid filled compartments, transversely partitioned by septa. In arthropods and mollusks, coelom is reduced or absent but it is present in the embryonic stage. Instead of coelom, they have space with blood and lymph, known as haemocoel. In these animals, the haemocoel is the primary body cavity while the coelom is the secondary body cavity.

Special Features of Coelom

  • Coelom is a secondary body cavity which is formed during embryonic development by splitting of mesoderm into two layers, a somatic layer and a splanchnic layer.t
  • Coelom is surrounded by the coelomic epithelium.
  • Reproductive organs arise from the walls of the coeloimic epithelium.
  • The perivisceral cavity or splanchnocoel is formed from the greater part of the coelom.
  • Coelom is filled with colorless coelomic fluid which provides space to house the viscera.
  • The excretory organs also open into the coelom.

Types of Coelom

There are the following three types of coelom, such as:

  • Acoelom
  • Pseudocoelom
  • True Coelom or Eucoelom
image of Coelom-types

Image showing different Coelom

Acoelom

In this case, coelomic fluid-filled cavity is absent but the space between the body wall and the gut is filled by connective tissue, known as mesenchyme. The animals without a coelomic fluid-filled body cavity between the body wall and digestive tract are known as acoelomates and the group of animals is referred to as acoelomata.

Examples: Acoelomata Phyla include:

  1. Gnathostomulida,
  2. Platyhelminthes and
  3. Nemertea,
  4. Gastrotricha,
  5. Cnidaria
  6. ctenophora
  7. Kinorhyncha.

Pseudocoelom

In this case, coelom is not originated by the mesodermal epithelium. Embryologically, the pseudocoelom is developed from the blastocoel of the embryo. The pseudocoelom is also known as false coelom. The pseudocoelom is not bounded by the peritoneum. The fluid of pseudocoelom contains pseudocoelocytes which has no relation with the reproductive and excretory organs.   Animals with a pseudocoelom are known as pseudocoelomates and the group of animals is referred to as pseudocoelomata.  Pseudocoelomata are also referred to as blastocoelomata or haemocoelomata.

Examples: According to Brusca and Brusca (2003),  pseudocoelomata phyla include:

  1. Rotifera
  2. Kinorhyncha
  3. Nematoda
  4. Nematomorpha
  5. Acanthocephala
  6. Loricifera

Coelom

In this case, the body cavity between the gut and body wall is bounded by peritoneum. Peritoneum is derived from the embryonic mesoderm. This type of cavity or coelom is known as eucoelom or true coelom. The coelomic fluid of true coelom contains amoebocytes. The animals with true coelom are known as coelomates and the group of animals is referred to as coelomata.

Examples: According to Brusca and Brusca(2003)  the following  phyla possess a true coelom:

  1. Annelida
  2. Onychophora
  3. Tardigrada
  4. Arthropoda
  5. Mollusca
  6. Phoronida
  7. Ectoprocta
  8. Brachiopoda
  9. Echinodermata
  10. Chaetognatha
  11. Hemichordata
  12. Chordata

Some Similarities Between Coelom and Pseudocoelom

  • Both coelom and pseudocoelom contain coelomic fluid.
  • Peritoneum is present in both coelom and pseudocoelom.
  • Both coelom and pseudocoelom are present in triploblastic animals that possess three germinal layers such as ectoderm, mesoderm and endoderm.
  • Both coelom and pseudocoelom contain alimentary canal.
  • Both coelom and pseudocoelom act as a hydroskeleton.

Difference between Coelom and Pseudocoelom

The following table shows the differences between coelom and pseudocoelom:

Coelom

Pseudocoelom

Coelom is a fluid filled true body cavity, present between the body wall and the digestive tract.

Pseudocoel is a fluid-filled body cavity, present between the mesoderm and the endoderm.

Coelom is also known as eucoelom or true coelom.

Pseudocoelom is also known as false coelom.

The animals with true coelom are known as coelomates and the group of animals is referred to as coelomata.

Animals with a pseudocoel are known as pseudocoelomates and the group of animals is referred to as pseudocoelomata.

Coelom or true body cavity is lined with peritoneum.

Pseudocoelom  or false body cavity is partially lined with peritoneum.

The coelom is developed by splitting of the mesoderm.

The pseudocoelom is developed from the blastocoel of the embryo.

In this case, the blood stream carries the nutrients.

In this case, the nutrient transportation occurs through osmosis and diffusion.

Coelom is present either in vertebrates or invertebrates.

Pseudocoelom is present only in invertebrates.

The organs present inside the coelom are Well-organized.

The organs present inside the pseudocoelom are less-organized.

Examples: Annelida, Arthropoda, Mollusca, Echinordermata, Hemichordata, Chordata, etc.

Examples: Nematoda, Entoprocta, Rotifera, Acenthocephala, and Gastrotrica, etc.

Functions/Significance of Coelom

  • Coelom protects the internal organs from external mechanical shocks or trauma by surrounding them.
  • It maintains the body shape and in earthworms and many other invertebrates, it functions as a hydrostatic skeleton to aid in locomotion.
  • Coelom helps to eliminate excretory waste products from the body after metabolic process.
  • Coelom is filled with coelomic fluid which act to separate the organs from the outer body.
  • In some organisms, coelom helps to perform respiration by keeping the body wall moist.
  • Coelom contains coelomocytes in the immune system of most invertebrates, acts as macrophage-like cells which are involved in phagocytosis, inflammation by enhancing immunity to the body and helps to destroy bacteria and other harmful organisms from the body.
  • It provides flexibility to the body and helps to transport of gaseous substances and nutritive materials from one part of the body to the other.
  • Coelomic cavity provides site for brooding of embryos, sperm and eggs during maturation.
  • Coelomic fluid contains nutrients which are distributed to all parts of the body in such a manner of a circulatory system.
  • Coelom acts as a reservoir for waste products and allows growth of internal organs. 

Concluding Remarks

Coelom makes the partition to separate biological systems that perform various major functions.  Acoelomates transport nutrients around the body through diffusion due to relatively small with less intricate body plan. In this case, diffusion is enough to perform transport function. The coelomates have complex body plan that need special transport system (circulatory system) to ship the nutrients around the body.  In this case, coelom is useful to separate circulatory system from abdominal organs.

Monocots and Dicots: Characteristics and Differences

Plants can be broadly divided into two types: flowering plants and non-flowering plants. In this case, flowering plant is also known as angiosperms while non-flowering plant is known as gymnosperms. Based on the nature of the embryo in the seed, angiosperms are again divided into the following two types:

  1. Monocotyledonous and
  2. Dicotyledonous plants

Monocotyledonous Plants

Monocotyledon is commonly known as monocot. They have seeds with one embryonic leaf or cotyledon; hence they are called monocotyledonous plants. This group contains about 60,000 species. Among them, the family Orchidaceae (orchids) contains more than 20,000 species. Besides these, the Poaceae (true grasses) is the most important family. Other prominent monocot families include Arecaceae (palms),  bananas, plantains (Musaceae), Liliaceae (lilies),  and Iridaceae (irises). This group includes different type of grains (rice, wheat, maize, etc.), forage grasses, sugarcane, the bamboos, etc.

Characteristics Features of Monocotyledonous Plants

  • In most cases, the plants are herbs and annual. Woody trees are very rare in this group.
  • No formation of tap root (main root) occurs in monocotyledonous plants. The primary root is destroyed, and fibrous adventitious roots develop.
  • The stem is generally unbranched. In some cases, unbranched stem bears nodes. The distance between the two nodes is called internodes, which may be solid (maize, sugarcane) or hollow (bamboo, grass). Again in some cases, the perennial plants develop underground stems such as oniom, arum, etc.
  • The leaf blade of monocotyledonous plants is elongated and remains angular with the surface of the earth and provided with sheathing leaf base.
  • Generally, in the axil of leaves, axillary bud and stipules are absent.
  • The leaves are provided with parallel variation and their margins are smooth.
  • The flowers are mainly trimerous because the number of members in each whorl is 3 or multiples of 3.
  • The seed coat and fruit coat are fused and generally, the seed are endospermic. The cotyledons may be flattened shield-like.
  • The smaller embryo is with only one cotyledon and the cotyledon is thin and scale-like.
  • Generally, hypogeal type of germination is noted in the monocotyledonous plants. In this type of germination, the seed never come out of the soil. The cotyledon is not green in color. The radical is covered by coleorhizae and the plumule is covered by coleoptiles. During seed germination, the radical and the plumule come out along with their covering i.e., coleorhizae and coleoptiles.
  • Due to the absence of cambium (lateral meristem), no secondary growth take place in the stem and root of monocotyledonous plants (exception: Dracaena, Yucca).
  • In most cases, the leaves are isobilateral. Both the leaf surfaces are similar and light falls equally on both the surfaces.

Dicotyledonous plants

The dicotyledonous plants are also known as dicots. These plants have two embryonic leaves or cotyledons. They contain about 200,000 species. Garden plants, shrubs and trees, broad-leafed flowering plants such as magnolias, daisies, roses, geraniums, cacti, peas, mint, hollyhocks and many more are dicots.

Characteristics Features of Dicotyledonous Plants

  • In most cases, the plants are either trees or shrubs and perennial. Plants may be annual and biannual also. Herbs are very few in this group.
  • Tap root is formed from the dicotyledonous plants.
  • The stem is branched and underground stems are formed in a few dicotyledonous plants.
  • The leaf blade of dicotyledonous plants are broad and remains parallel with the earth`s surface. The leaves are not provided with sheathing leaf bases.
  • In the axil of leaves (i.e., junction of stem and petiole) lies axillary bud and stipules.
  • The leaves are provided with reticulate venation and their margins are broken.
  • The flowers are mainly tetra or pentamerous because the number of members in each whorls is 4 to 5.
  • The seed coat and fruit coat are not fused and generally, the seeds are non-endospermic.
  • The comparatively large embryo is with two cotyledons and the cotyledon is thick.
  • The germination of seed may be epigeal or hypogeal. Sometimes viviparous type of germination is noted. In epigeal germination, the cotyledons come out above the soil surface. Cotyledons are green in color. In this case, the radical and plumule comes out directly without any covering (i.e., coleorhizae and coleoptiles).
  • Due to presence of cambium (lateral meristem), secondary growth take places in the stem and root.
  • The leaves are dorsiventral.  The upper and lower surfaces of the leaves are differentiable. Sunlight falls directly on the upper surface.
image of Germination process of dicot

Germination Process of Dicots

image of Germination process of monocot

Germination Process of Monocots

image of Monocot and dicot flowers

Image showing monocots and dicots flowers

image of Root system of monocot and dicot

Root system of monoctots and dicots

Difference Between Monocotyledonous and Dicotyledonous Plants

Dicotyledonous Plants

Monocotyledonous Plants

Nature

Cell

Dicotyledonous Plants are generally trees; few are herbs and shrubs.

Monocotyledonous Plants are generally herbs. Few are trees.

They are branched trees.

They are unbranched trees.

Perennial plants; very few plants possess underground stems.

Perennial plants, underground stems are present which helps in penetration in the soil.

Root

Cell

Main roots or true roots are formed, they are generally branched.

Main root or true root does not form. In most cases, the radical degenerates and later on produces adventitious roots from the base of the stem.

The number of radial vascular bundles lies between 2-6.

The number of radial vascular bundles are more than 6.

Xylem vessels are polygonal in shape.

Xylem vessels are oval in shape.

In most cases pith is absent. If present, it occupies a smaller area in the center.

The well developed pith is present and it occupies a larger area in the center.

In this case,  secondary growth occurs due to the presence of cambium.

Secondary growth does not occur due to the absence of cambium.

Stem

Cell

Stem is branched.

Stem is generally unbranched and jointed in nature.

The vascular bundles are conjoint, collateral and open. It is composed of xylem, phloem and cambium. Vascular bundles may be bi-collateral.

The vascular bundles are conjoint collateral and closed. It is composed of only xylem and phloem, cambium is absent.

The number of vascular bundles is lesser. The vascular bundles are arranged within a ring.

The number of vascular bundles is greater. The vascular bundles are scattered within the ground tissue.

The epidermis (without cuticle layer) generally possesses multicellular hairs (stem hairs). 

The epiblema (without cuticle layer) is generally without hairs. 

Hypodermis is collenchymatous type.

Hypodermis is parenchymatous or sclerenchymatius type with thick walled.  

Each vascular bundle is generally with bundle cap (sclerenchymatous). Bundle sheath is absent.

A bundle sheath surrounds each vascular bundle. Bundle cap is absent.

Due to the presence of cambium, secondary growth occurs

Secondary growth does not take place due to the absence of cambium.

Annual rings are formed due to secondary growth.

Due to non occurance of secondary growth, annual rings are not formed.

Leaf

Cell

Leaves are broad, remains parallel with the earth`s surface. The leaves are not provided with sheathing leaf bases.

Leaves are elongated, remains angular with the earth`s surface and they are provided with sheathing leaf bases.

The leaf margins are broken.

The leaf margins are entire (smooth).

In the axil of leaves, axillary bud lies.

In the axil of leaves, no formation of axillary bud.

The leaves are provided with reticulate venetion (exception: Calophyllum inophyllum). 

The leaves are provided with parallel vanation (exception: Arum).

Leaves are mainly dorsiventral. The upper surface and the lower surface are not similar.

Leaves are mainly isobilateral. The upper and lower surface are similar.

The mesophyll tissue of the leaf is differentiated into upper elongated palisade parenchyma and lower more or less round spongy parenchyma.

The mesophyll tissue is composed of only one type of similar parenchyma cells like spongy parenchyma.

Stomata is mainly present on the lower epidermis.

Stomata is present on both the upper and lower epidermis.

Flower

Cell

The flowers are mainly pentamerous or tetramerous or multiples of 5.

The flowers are mainly trimerous or multiples of 3.

Inflorescence are of different types.

Inflorescence are of different types. But in case of grass like plants, spikelet inflorescence is present.

Seeds

Cell

Embryo possesses two thick cotyledons.

Embryo possesses only one thin and scaly cotyledon.

Seed coat and fruit coat remains separate, not fused together.

Seed and fruit coat fused together.

The nature of the seed is generally non endospermic.

The nature of the seed is generally endospermic.

The embryo of the seed is larger in size.

The embryo of the seed is smaller in size.

The radical and plumule is not covered by means of coleorhizae and coleoptyle respectively.

The radical and plumule is covered by means of coleorhizae and coleoptyle respectively.

Germination

Cell

Germination mainly hypogeal or epigeal.

Germination is mainly hypogeal.

Radicle and plumule comes out directly without any covering (i.e., coleorhizae and coleoptyle).

During seed germination the radical and plumule comes out along with their covering coleorhizae and coleoptiles.

Radicle and plumule is not covered by means of membranes like coleorhizae and coileoptile, respectively.

Radicle and plumule is  covered by means of membranes like coleorhizae and coileoptile, respectively.

In epigeal germination, the cotyledons come out above the soil surface and take green coloration.

The cotyledon never comes out above the soil surface during seed germination.

The radical penetrates the soil and forms main roots with permanent root system.

The radical after its preliminary growth within the soil is destroyed and from the region fibrous adventitious roots are formed. 

The position of embryo is terminal.

The position of embryo is lateral.

Examples:

Cell

Dicotyledonous plants: Sunflower: Helianthus annuusMango: Mangifera indica

Monocotyledonous plants: Paddy: Oryza sativa, Banana: Musa paradisiaca, etc.

image of Difference between monocot and dicot plants

Image showing difference between monocots and dicots

Spirogyra: Characteristics, Structure and Reproduction

Spirogyras are common free-floating freshwater algae that inhabit ponds, pools, tanks, lakes, ditches, etc.  The word ‘Spirogyra’ is derived from the two Greek words, ‘Speria’, meaning coil, and ‘gyras’ meaning twisted. Spirogyra has many common names, including blanket weed, water silk, mermaid`s tresses, etc. It grows up to several centimeters in length and 10-100 μm in width. There are about 400 known species of Spirogyra worldwide. They are filamentous and slippery in natures due to the presence of external mucilaginous sheath; hence, they are called pond scum or pond silk. Some of the species of Spirogyra (Spirogyra adnata, S. jogensis) bear holdfast or haptera by which they remain attached to the substratum.

Systematic Position

Division: Chlorophyta

Class: Chlorophyceae

Order: Zygnematales

Family: Zygnemataceae

Genus: Spirogyra

Species: Spirogyra maxima, S. negnecta, S. elongate, S. adnata, S. nitida, etc.

Identifying Characteristics of Spirogyra

  • They have a multi-cellular filamentous body with a mucilaginous sheath.
  • They bear 2-10 spiral and ribbon-shaped chloroplasts with many pyrenoids.
  • The cell wall is composed of pectin and cellulose.
  • It reproduces vegetatively and sexually.
  • Under lower temperature, vegetative reproduction occurs.
  • It inhabits slow running water bodies and shows the mass of long shining silky filaments in running water; hence, it is known as pond silk.

Spirogyra is un-branched green algae that belong to the class Chlorophyceae under order Zygnematales. It consists of long identical cylindrical cells situated one above the other, without any differentiation into base and apex. It bears 2-10 spiral ribbon-shaped chloroplasts with many pyrenoids, which are the distinctive characteristic features of Spirogyra.

Their margins may be smooth or serrated. Spirogyra floats freely in masses over the surface of the water and is moved in favor of water current. But Spirogyra adnata remain attached to the substratum by means of holdfast. They are autotrophic in nature because they contain photosynthetic pigments and perform photosynthesis for producing their own food.

Structure of the Vegetative Body

The plant body is un-branched filamentous, green-colored with cylindrical cells placed end to end. In free-floating species, there is no basal differentiation, but in some sedentary species, the basal cell is modified into a haptera or holdfast, which is the organ of attachment. The lateral cell wall is stratified and three-layered, the inner two layers are composed of cellulose, and the outermost layer is composed of pectose. The cross walls are also three-layered, the middle lamella is composed of pectose bounded on either side by layers of pectin, in some other species, there is an additional annular ingrowth of cellulose along with the cross wall, giving rise to the so-called replicate wall which again is of five types such as plane, replicate, semi-replicate, colligate (cross wall looks like a short H-like piece) and unduliseptate, respectively. 

A vacuolated, granular protoplast is present in each cell in the form of a thin lining along the cell wall and is termed as the primordial utricle. The vacuole is separated from the surrounding cytoplasm by a semi-permeable membrane, the tonoplast. The eukaryotic nucleus with a distinct nucleolus remains embedded within the cytoplasm or resides at the center being supported by the cytoplasmic strands. Each cell bears a varying number of ribbon-shaped, spiral chloroplasts with either serrated or smooth edges. In the chloroplasts, dense, highly refractive, granular, protein body surrounded by starch called pyrenoids at short intervals. In some cases, pyrenoid like bodies without starch sheath is also present called protopyrenoids.

image of Vegetative structure of Spirogyra

Vegetative structure of Spirogyra

They also contain other cellular organelles such as endoplasmic reticulum, Golgi bodies (dictyosomes), mitochondria, ribosomes, etc. which remains scattered within the cytoplasm.

Reproduction of Spirogyra

They increase in length of the filament takes place by ordinary cell divisions and by subsequent growth of individual cells, each of which may divide further. Spirogyra reproduces vegetatively and sexually. In this case, asexual reproduction is absent.

Vegetative Reproduction of Spirogyra

Vegetative reproduction in Spirogyra takes place by means of fragmentation. This is performed by softening of the cross wall between the two adjacent cells, as a result of which each part or piece of the broken filament grows out into a filament by repeated cell divisions, or by the accidental breaking of the filament by external mechanical injury.

image of Fragmentation of Spirogyra

Fragmentation of Spirogyra

Sexual Reproduction of Spirogyra

Spirogyra may be monoecious or dioecious. Sexual reproduction takes place by conjugation of two morphologically identical gametes and each of which is called a gametangium.  The sexual reproduction shows physiological anisogamy, as out of the two isogametes, one is motile, and the other one is non-motile. Sexual reproduction occurs at different times of the year according to species. Two types of conjugation are found in Spirogyra, which is described below:

Scalariform or Ladder Like Conjugation of Spirogyra

This type of conjugation takes place in the heterothallic species. In such cases, two filaments of opposite strains, i.e., + and – approach each other and lie in close association throughout their entire length. Now, the cells of the two associated filaments send out protuberances from the walls of the cells in opposite directions. These protuberances in due course come in contact with each other. Finally, the end walls of these protuberances are dissolved, forming a continuous passage, i.e., the conjugation tube. The protoplasts of the conjugating cells begin to shrink to form gametes. The gamete of the cells of one filament shows amoeboid movement and move across the conjugation tube to the cells of the other filament. Now, the two gametes undergo fusion to form a dark-colored diploid zygospore body. The cell of the two filaments, along with the conjugation tube, gives a ladder-like appearance; hence, this type of conjugation is termed as scalariform or ladder-like conjugation.

Scalariform Conjugation of Spirogyra

Lateral or Chain Like Conjugation of Spirogyra

This type of conjugation takes place between the adjacent cells of the same filament, i.e., in the homothallic species. Lateral walls of the conjugating cells of the same filament develop protuberances on either side of the cross wall. As the protuberance increases, the cross wall fails to maintain its connection with the lateral wall, whereby a passage is established between the two cells. Through this side passage, the shrunk protoplasts, i.e., the gamete (aplanogamete) of one cell, migrates into the adjacent cell and fuses with the other gamete. Fusion results in the formation of a diploid zygospore. This type of conjugation is termed as the lateral conjugation.

image of Stages of lateral conjugation

Stages of Lateral Conjugation of Spirogyra

Germination of Gamete

The zygospore surrounds itself with a three-layered thick wall and is resistant to cold and drought. The zygospore generally sinks to the bottom of the water and after remaining dormant for some time, undergoes germination. The diploid nucleus undergoes meiosis forming four haploid nuclei, three of which degenerate, and one remains functional.

image of germination of zygote

The wall ruptures, and the innermost cellulose layer emerges out in the form of a cylindrical germ tube. The germ tube undergoes transverse division to form 2 celled spirogyral filaments, which by repeated division gives rise to the cylindrical multi-cellular spirogyral filament. The functional haploid nucleus constitutes the nucleus of the haploid zygospore.

Parthenogenesis

Parthenogenesis may occur in Spirogyra by the development of perthenospore or azygospore. These are formed at the condition when the gametes fail to fuse. The gametes secrete a thick wall around it to become a perthenospore, which after a period of rest, germinate to form a new filament.

Life Cycle of Spirogyra

In the life cycle of Spirogyra, alternation of haploid (n) generation and diploid (2n) generation is noted. The haploid (n) phase is long, but the diploid (2n) phase is very short-lived. The diploid phase is restricted within the zygospore only.

image of Life cycle of Spirogyra

Concluding Remarks

The Spirogyra produces food matters by means of photosynthesis, and many aquatic animals use them as food. Spirogyra is used as fish`s food. Dried Spirogyra used in the preparation of soups. Spirogyra contains lots of vitamins A and E. It is cultured in garden tanks for ornamental purposes. Spirogyra is also used in the aquarium. Besides these, Spirogyra spoils the water of drinking tanks. If it grows in abundance, Spirogyra, create disturbances in swimming and fishing. 

Reproduction in algae

Algae are a diverse group of eukaryotic cellular or multi-cellular organisms. They inhabit both in aquatic and terrestrial environments. Some occur in moist stones, wood, soils, on ice, or snow. They have photosynthetic pigments that perform photosynthesis, produce oxygen, and remove at least half of the total carbon dioxide from the earth`s atmosphere.

Algae perform reproduction in various ways. It can be vegetative, asexual, or sexual. In this case, vegetative propagation occurs through fragmentation; asexual reproduction occurs by forming different spores and binary fission, while sexual reproduction occurs by fusion of two haploid gametes. Generally, many environmental factors influence the reproduction of algae.

You might also read: Structure of Algae

The following three main types of reproduction process occur in algae:

  1. Vegetative Reproduction
  2. Asexual Reproduction and
  3. Sexual Reproduction
image of Reproduction types in Algae

Reproduction types

Vegetative Reproduction

It is simple and the most common process of reproduction in algae. By this process, vegetative parts of thallus divide into small fragments, and each part, later on, gives rise to a new plant. This process is termed as fragmentation. This type of reproduction occurs in Spirogyra, Ulothrix, etc. In this process, no alternation of generations occurs, and it does not require any spore formation.

The vegetative reproduction in algae is of the following types:

Fission or Cell division

In this process, unicellular algae reproduce new individuals. It is a very simple method, and often it is known as binary fusion. During reproduction by this process, mitosis occurs in the vegetative cell and produces two daughter cells and finally acts as new individuals. This type of reproduction occurs in Synechococcus, Chlamydomonas, diatoms, etc.

Fragmentation

This process occurs in multi-cellular filamentous algae. In this process, the thallus breaks down into many fragments; among them, each fragment gives rise to a new individual. Generally, the fragmentation process occurs accidentally or by any other mechanical injury. This type of vegetative reproduction occurs in Ulothrix, Spirogyra, Zygnema, Oedogonium, Cylindospermum, etc.

image of Fragmentation in Spirogyra

Fragmentation in Spirogyra

Hormogonia

This type of reproduction occurs in blue-green algae such as Oscillatoria, Nostoc, Cylindosporium, etc. In this process, trichome breaks down within the sheath into many segments, and each segment forms a new individual. In this case, each segment is known as hormogonium.

Adventitious Branches

In many large thalloid algae, adventitious branches form and detach from the mother`s body to develop new individuals. In this case, internodes, stolons, etc. form adventitious branches. This type of reproductions occurs in Fucus, Dictyota, Chara, Cladophora, etc

Bulbils

Bulbils are the tuber-like outgrowth, which occurs due to the storage of food at the tip of the rhizoids and lower nodes of the plants. During reproduction, bulbils detach from the plant body and give rise to new individuals. This type of reproduction occurs in Charophyta, such as chara, Lamprothamnium, Nitellopsis, etc.

image of Bulbils

Amylum stars

In Chara, amylum stars occur and detach from the plant body; they develop into new plants. The cells of the lower nodes of plants store starch and form star-shaped aggregation, which is known as amylum stars.

Budding

The bud-like outgrowth occurs in some algae, such as Protosiphon. In this process, the nucleus from the parent body divides by mitosis and produces daughter nucleus, which migrates to develop bud. The newly formed buds detach from the parent body and live independently as a new individual.

Asexual Reproduction

Asexual reproduction occurs by forming a specific type of spores. In this process, some cells or protoplasm of few cells of the plant divide to create a small-sized structure, the spore. Each spore is germinated and liberated from the mother cell and gives rise to a new plant. It generally takes place by the following method:

By Forming Zoospores

In this process, under favorable conditions, biflagellate, tetra-flagellate, multi-flagellate, naked, motile zoospores are formed and on bursting of mother cells (sporangium) come out and give rise to the new plant. The zoospores may be either haploid or diploid, which are formed within the zoosporangium. In this case, biflagellate zoospores occur in Ulothrix, Chlamydomonas, Ectocarpus, etc. while tetra-flagellate zoospores occur in Ulothrix and Oedogonium, Vaucheria, multi-flagellate zoospores occur.

By Forming Aplanospores

In this case, protoplasm gets separated from the cell wall to form one or more thin-walled, non-nucleated aplanospores, and each aplanospore gives rise to a new plant. In this case, aplanospores are non-motile spores that occur inside sporangium under unfavorable conditions. This process of reproduction occurs in Chlorella, Ulothrix, Scenedesmus, Microspora, Pediastram, Sphaerella, etc.

By Forming Hypnospores

In this process, under unfavorable conditions, the protoplasm of the cells separate from the cell wall and collects in the center. These are non-motile with thickened walls and abundant food reserves, known as hypnospores. They give birth to a new plant on the commencement of favorable conditions. This process occurs in Vaucheria.

By Forming Akinetes

In certain filamentous algae, the vegetative cells become thick and develop into spore-like structures with abundant food reserves, known as akinetes. Akinets are formed under unfavorable conditions, and on commencement of favorable conditions, each akinete develops into a new plant. This type of process occurs in Cladophora, Oedogonium, etc.

By Forming Endospores

In most of the members of Myxophyceae, the development of a large number of endospores takes place inside the mother cell. Endospores germinate directly and give rise to a new plant under favorable conditions. In this case, the mother protoplasm divides and forms small spores. These spores are known as conidia or gonidia. This type of reproduction occurs in Dermocarpa.

Pamella Stage

Under dry conditions, zoospores or aplanospores don't come outside the mother cell but get encompassed by an adhesive sheath. The division proceeds with the outcome; they take the form of a colony. This is known as the pamella stage. Under suitable conditions, they turn out. Every zoospore or aplanospore offers ascend to another plant. This procedure happens in Chlamydomonas sp.

By Autospores

In some algae, the undeveloped spores grow into new plants within the mother cell. This type of reproduction occurs in Chlorococcus.

By Cysts

In some algae, under unfavorable conditions and abundant food supply, the thallus divides into multinucleate and thick-walled smaller segments, which are termed as cysts. In favorable conditions, cysts give rise to new plants. It occurs in Vaucheria sp.

Sexual Reproduction

All the members of the algae reproduce sexually except the representatives of the class Cyanophyceae. Sexual reproduction occurs by fusion of gametes. These gametes grow within gametangia. Sexual reproductions are of the following five types:

Isogamous

In some algae, the gametes and their size and external morphology are alike. In this type of reproduction, Positive (+) and negative (-) strain gametes combine together to form zygospore or zygotes. These gametes are known as isogametes, which may be motile (zoogametes) or non-motile. In this case, isogametes of Chlamydomonas and Ulothrix are motile, while isogametes (aplanogametes) in Spirogyra are non-motile type.

image of Isogamous

Isogamous in Chlamydomonas

Anisogamous

In some algae, the gametes and their external morphology are alike, but their behavior and size vary. In this case, the large-sized and passive gamete is known as macrogamete, while the smaller and active gamete is known as microgamete. These two gametes fuse and produce a zygote, which later on undergoes mitotic division to form new individuals. This type of reproduction occurs in Chlamydomonas.

image of Anisogamous in Spirogyra

Anisogamous in Spirogyra

Oogamous

In this type of sexual reproduction, fertilization occurs between small, biflagellate, or multi-flagellate, active male gamete with large non-flagellate, passive female gamete, and form zygote (oospore). In this case, male gametes grow within antheridium while the female gametes within the oogonium. This type of reproduction occurs in Oedogonium, Chlamydomonas, Vaucheria, Laminaria, Chara, Poly-siphonia, Sargassum, Batrachospermum, etc.

image of Oogamous

Oogamous in Chlamydomonas

Concluding Remarks

Algae act as a significant component of the food chain in the aquatic ecosystem as a primary producer. Algae have a huge source of carbohydrates, lipids, proteins, and vitamins A, B, C, and E, etc. They also contain very important minerals sources for animals such as iron, magnesium, potassium, manganese, calcium, and zinc. Some algae use as human foods such as agar is used as commercial products while Graularia, Gelidium, etc. are used to grow microbes for making ice creams and jellies. Chlorella and Spirullina contain rich proteins that are used as food supplements.To make the aquatic ecosystem friendly, we should take conservation measures for protecting the algae species. 

Structure of Algae

Algae are photosynthetic microorganisms that perform photosynthesis and produce oxygen (O2) and consume carbon dioxide (CO2) from the atmosphere. During photosynthesis, algae produce at least half of the oxygen in Earth’s atmosphere. Most of the algae inhabit the aquatic environment, either freshwater or marine habitats. They can also occupy on rocks, soils, vegetation, or moist terrestrial habitats. Some marine algae use as seafood, while some algae produce toxins.

Vegetative Structure of Algae

The vegetative structures (thallus) of algae vary from species to species. They may be simple unicellular to complex multi-cellular. Their size ranges from small as less than 2 micrometers (Micromonas) to large as 30-60 meters long (Macocyctis, a type of marine algae). The following are the diverse vegetative structures of algae:

Unicellular motile: The body consists of a single cell. In these algae, movement takes place by flagella, such as Chlamydomonas.

image of Clamydomonas

Chlamydomonas.

Unicellular non-motile: They are single-celled round algae. They exist as solitary or in the group within mucilage covering. Examples: Chlorella, Gloeocapsa, etc.

Motile colonial: In this case, definite numbers of unicellular algae form colony and they are motile. This type of colony is known as a motile colony. Examples: Volvox.

image of Volvox

Volvox

Non-motile Colonial: In colonial forms, the number of unicellular algae is indefinite, and they are non-motile such as Scenedesmus, Hydrodictyon (water net), etc.

image of Scenedesmus

Scenedesmus

Palmelloid type: In this case, algae cells are surrounded by a viscous mucilaginous substance such as Teyraspora, Aphanothce, etc.

Dendroid type: In this case, cells produce a mucilaginous substance, and they are folded in such a manner that they look like branches of a plant such as Prasino cladus.

Filamentous: These types of algae are of two types: Among them, some are simple un-branched filamentous such as Ulothrix, Spirogyra, and some are simple branched filamentous such as Cladophora.

image of Spirogyra

Spirogyra

Heterotrichous: Thallus of the algae consists of the following two parts: main shoot or trichome, which runs horizontally, termed as a prostrate system and a vertical erect trichome or shoot termed as erect system. This type of body is known as a heterotrichous form.

Parenchymatous: In these algae, cell division takes place on different sides, with the result that they become parenchymatous such as Ulva.

Siphonous: These algae consist of the multinucleate tube-like cells having no septa such as Vaucheria, Polysiphoni, etc.

Nodous: In this case, algae bodies contain nodal and intermodal areas such as Chara.

image of Chara

Chara

Complex: In this case, algae look like multi-cellular plants — the body divisible into holdfast, stipe, and frod, such as Sargassum, Laminaria.

image of Sargassum

Sargassum

Cellular Structure of Algae

The algal cells consist of the following structures:

Cell wall

Most of the algal cells have a cell wall. Some flagellated algae are lacking a cell wall. Algae cell-wall consists of two layers: inner microfilamentous and outer gelatinous irregular layer. Chemically, the cell wall is composed of cellulose, pectin, mucilage like a carbohydrate. It also contains other substances like alginic acid, calcium carbonate, fucoidan, fucin, silica, etc. The thickness of the cell wall varies with variously oriented in a granular matrix. In some cases, cell wall bears stored protein. Unicellular diatom algae have strong, rigid, siliceous ornamental, two valves cell walls. The siliceous ornamental cell wall is called frustule. The cell wall of the members of Cyanophyceae contains mucopeptide. In certain algae such as Gymnodinium and Pyramimonas, a true cell wall is absent. In this case, the cell is bounded by a membrane, known as a pellicle.

image of Euglena

Euglena Cell Structure

Plasma membrane

It occurs below the cell wall. The structure of the algal plasma membrane is like other eukaryotic cells. Some filamentous algae lack cell wall having stout and strong plasma membrane. This type of plasma membrane is known as periplast.

Protoplast

The eukaryotic algal protoplasm consists of one or more nucleus and cytoplasm. The cytoplasm is divided into cytosol and cell organelles. The protoplasm is bounded by lipoproteinaceous cell membrane, which is fluid mosaic in nature. The membrane is elastic and very thin, with selectively permeable in nature. It performs to control the passage of materials in and out of the cells. The algae have well organized spherical or elliptical shaped nucleus, which is surrounded by a distinct and double layer nuclear membrane. In this case, the outer membrane is attached to the endoplasmic reticulum(ER), while the inner membrane contains a matrix or karyolymph with chromatin reticulum.

The numbers of nucleoli or endosomes vary in different algal cells with varying numbers of chromosomes. Porphyra linearis contains the lowest numbers of chromosomes (n=2), while the highest numbers of chromosomes are found in Netrium digitali (n=592).

Cells Organelles

Like other eukaryotic cells, eukaryotic algal cells have membrane-bound cell organelles like chloroplasts, mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum, and in some cases, eyespot or stigma. In the prokaryotic algal cell, the nucleus is not surrounded by a membrane. In this case, the protoplasm is separated by photosynthetic pigments containing outer peripheral chromoplasm and colorless inner centroplasm.

Nucleus

Most of the algal cells are uni-nucleated, but in some cases, they are multinucleated.

Plastids

Algal cells have colorful plastids, the chloroplasts. It is the prominent feature which is bounded by double-membrane structure; their number and shape vary in different species.

The chloroplasts of algae have various forms and shapes; in algae, eight main types are recognized: discoid (Chara), cup-shaped (Volvox), parietal, spiral (Spirogyra), C-shaped or girdle shaped (Ulothrix), reticulate (Oedogonium), ribbed (Volvocales), and stellate (Zygnema).

The chloroplast contains the following three major structural regions:

Envelope: It is the two membranous enclosed spaces.

Stroma: It is a very important structure that helps to store starch and contains enzymes for protein synthesis and metabolism.

Internal lamellar membranes: It is a highly organized membrane that contains different types of pigments for capturing energy. This lamellar system forms sac-like discs or thylakoid, which are stacked together and form grana. This thylakoid provides space for the chlorophyll a and other accessory pigments.

In most cases, chloroplasts have a glycoprotein structure, known as a pyrenoid. Chloroplasts of all kinds of algae have photosynthetic chlorophyll pigments which provide the actual color of the thallus.

In algae, five types of chlorophylls are found (Chlorophyll a, b, c, d, and e). Among them, chlorophyll a is found in all groups of algae, while chlorophyll b is present only in Chlorophyceae. Chlorophyll c is located in the representatives of Bacillariophyceae Cryptophyceae, Chryso-phyceae, and Phaeophyceae, chlorophyll d is seen in some red algae, and chlorophyll e is found in certain Xanthophyceae.

Besides chlorophyll, they also show various carotenoid pigments that impart different colors to algae as blue phycocyanin, yellow-brown fucoxanthin, brown phaeophycin, red phycoerythrin, etc. In this case, carotenoids are composed of carotenes and xanthophylls. In algal cells, the following five types of carotenes are found: α-carotene, β-carotene, c-carotene, e- carotene, and flavacene.

Besides carotene, several types of xanthophylls are found, such as lutein, violaxanthin, neoxanthin, fucoxanthin, myxoxanthophyll, myxoxanthin and oscilloxan. In this case, fucoxanthin is the main xanthophyll pigment.

In algae, phycobilins are also found as accessory pigments. They act as bilioproteins and provide either blue (phycocyanin) or red (phycoerythrin) in color. These types of pigments are found only in Rhodophyceae and Cyanophyceae, which absorb and transfer the light energy to the reaction center. Chloroplasts also contain proteinaceous bodies, known as pyrenoids. They perform to synthesize and store starch.

Reserve food

The staple reserve food of algal cells is carbohydrate. Different algae have different types of reserve carbohydrates. Green algae (Chlorophyceae) have starch, brown algae (Phaeophycea) have laminarin, and mannitol, yellow-golden (Chlorophytes) algae have volutin, red algae (Rhodophyceae) have Floridian starch as reserve food. Besides, these algae have oil and fat, leucocin, paramylum as reserve food.

Concluding Remarks

Algae are a diverse group and very familiar to most people. They play an important role in the freshwater environment and act as the base for the aquatic food chain supporting all fisheries in the inland and oceans. They also remove excess nutrients and pollutants from the water controlling eutrophication. In some cases, excessive growth of algae can cause harmful effects to aquatic environments.

Algae: Characteristics, Types and Its Classification

Algae are the green slimy blanket which covers the rock surface or the top of the ponds or a poorly kept aquarium that have the ability to conduct photosynthesis. The algae belong to the subphylum Thallophyta of the kingdom Protista in modern classification of organisms. Latin ‘alga’ means seaweed. These are eukaryotic organisms, autotrophic in nature as have chlorophyll in their cells. During photosynthesis, they produce oxygen with help of light energy from the sun and generate carbohydrates.

They possess many types of life cycles and their size range from microscopic Micromonas species to giant kelps. In this case, kelps reach up to 60 meters (200 feet) in length.

More than 30000 species of algae have been identified. Most algae are aquatic but some grow in semi-aquatic and terrestrial environments. Many algae live as endophytes in plant or animal tissue and many grow on plant or animal as epiphytes. Some of them make asymbiotic relationship with fungi and exist as lichen. Green algae, brown algae, red algae, golden-yellow algae are main types of algae. The study of algae is known as Phycology.

Characteristics of Algae

  • Algae are unicellular, colonial or large multi-cellular organisms.
  • The multi-cellular algae develop specialized tissues but they lack the true stems, leaves, or roots.
  • Most algae are aquatic but some are semi-aquatic and terrestrial.
  • Cells contain photosynthetic chlorophyll and other pigments.
  • Algae cell wall composed of cellulose and pectin.
  • With a few exceptions, most algae are autotrophic; they do not have vascular tissues.
  • Most algae store carbohydrate as reserve food, few members contains alcohol, fat or oil as reserve food.
  • They reproduce through vegetative, asexual and sexual methods. Asexual reproduction occurs by fragmentation or producing spores. Sexual reproduction can be isogamous, anisogamous or oogamous types.
  • Gametangia (reproductive organ) always single celled, if multi-celled, do not cover with sterile cell layer.
  • Zygote develops by either mitosis or meiosis cell division. Zygote never form embryo. 
  • Meiosis cell division is seen in different stages of life cycle.

Classification of Algae

image of Algae classification at a glance

Outline of Algal Classification

Division-1: Chlorophyta (Green Algae)

  • This group contains about 7000 species, among them, most occur in freshwater and some others in marine environment.
  • They contain pigments like chlorophyll a and chlorophyll b.
  • They also possess accessory pigments like carotenoids and xanthophylls.
  • Cell wall consists of cellulose, hemicellulose, and calcium carbonate in some species.
  • They store food as starch inside the chloroplast. 
  • Mitochondria contain flattened cristae.
  • They contain two or more flagella which do not bear tubular hairs (mastigonemes).

Class-1: Chlorophyceae

  • They have unicellular, colonial, filamentous, or multicellular body.
  • They contain pigments like chlorophyll a, chlorophyll b and beta-carotene.
  • They bear one or more storage bodies, known as pyrenoids located in the chloroplast.
  • The cell wall is rigid, composed of cellulose and pectose.
  • Asexual reproduction occurs by zoospores, aplanospores, hepnospores, akinetes, Palmella stage, etc.
  • Sexual reproduction is anisogamous, isogamous, or oogamous types.
  • Most of them inhabit in freshwater environment.

Examplses:  Chlorella, Chlamydomonas, Oedogonium,  Dunaliella,  Volvox, etc.

This class includes the following orders:

Order-1: Dunaliellales (e.g. Dunaliella)

Order-2: Chlamydomonadales (e.g. Volvox, Chlamydomonas)

Order-3: Chlorococcales

Order-3: Oedogoniales (e.g. Oedogonium)

Order-4: Sphaeropleales

Order-5: Chaetophorales

Order-6: Microsporales

Order-7: Tetrasporales (e.g. Tetraspora)

Class-2: Charophyceae

  • They are also known as stoneworts" and "brittleworts".
  • They are commonly found in freshwater environment.
  • They have unicellular, filamentous, colonial, or multicellular and plantlike bodies.
  • Many species bear flagellated cells.
  • They can store starch in characteristic plastids.

Examples: Stonewort (Chara), filamentous (Spirogyra) and desmids.

This class includes single order:

Order-1:  Charales

Class-3: Pleurastrophyceae

  • They inhabit in both freshwater and marine habitats.
  • They include coccoid, sarcinoid, and filamentous algae.
  • They reproduce asexually by autospores or by biflagellate, flattened zoospores.
  • Sexual reproduction is unknown among the representatives of Pleurastrophyceae.
  • They possess a counter-clockwise orientation of the basal bodies and an unusual mitotic spindle.

Examples: Marine flagellate (Tetraselmis).

Class-4: Prasinophyceae (Micromonadophyceae)

  • They have both unicellular and colonial forms.
  • They possess one, two, four, or eight flagella, with or without cell walls, scales, thecae, or loricae.
  • They are mostly motile and photosynthetic algae containing pigments like chlorophyll a, and chlorophyll b.
  • They are mainly marine algae, but some are also found in brackish- and freshwater habitats.
  • A few algae inhabit benthic region, with both coccoid and colonial forms.
  • There are about 180 known species under 13 genera.

Examples: Micromonas, Ostreococcus, Pyramimonas, etc.

Class-5: Ulvophyceae

  • They are marine algae with a variety of shapes.
  • The body consists of a few cells with thin sheaths, long filaments.
  • They possess two or more apical flagella, if present.
  • They perform photosynthesis due to presence of chlorophyll in their body.
  • Alternation of generations occurs in their life cycle.

Examples:  Sea lettuce (Ulva), Acetabularia, Caulerpa, Monostroma, etc.

Division-2: Chromophyta

  • They contain chlorophyll a and chlorophyll c.
  • They also contain accessory pigments such as Carentoids, xanthophylls.
  • They store foods as oils or carbohydrates laminarin.
  • If present, flagellum possesses hair like projection.  
  • Mitochondria contain tubular cristae.
  • Mucous organelles are common.

Class-1: Bacillariophyceae (Diatoms)

  • They primarily inhabit in freshwater, marine, and soil environments.
  • There are about 12,000 to 15,000 living species.
  • The cell contains a silica cell wall which is known as frustules. In this case, frustule is made up of two valves called thecae.
  • They contain  pigments like  chlorophylls a and chlorophyll c with accessory pigments such as  beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin.

Examples: Cyclotella, Thalassiosira ,  Bacillaria, Navicula, Nitzschia, etc.

Class-2: Chrysophyceae (Golden Algae)

  • They are commonly known as golden algae.
  • They inhabit in freshwater environment.
  • They are unicellular or colonial organisms.
  • The photosynthetic pigments are chlorophyll a, chlorophyll c and fucoxanthin.
  • They store energy as carbohydrate and oil droplets.
  • The cell contains silica deposition vesicles.
  • They possess apical flagella which is unequal in length.
  • They perform haplontic type life cycle.
  • Reproductions occur through binary fission, sporogenesis, etc.
  • This class contains about 1200 known species.

Examples: Chrysamoeba,  Lagynion, Chrysocapsa, Ochromonas, etc

Class-3: Dictyochophyceae

  • They are a small group of unicellular heterokont algae.  
  • They contain golden-brown chloroplast.
  • They have a long and wing like flagella.
  • They are predominantly marine algae.
  • Bases of flagella attach directly to nucleus.
  • This class contains only 25 described species.
This class includes the following orders:

Order-1: Pedinellales

Order-2: Olisthodiscales

Order-3: Sulcochrysidales

Order-4: Pelagomonadales

Order-5: Sarcinochrysidales

Order-6: Florenciellales 

Examples: Apedinella,  Mesopedinella, Parapedinella, Actinomonas, Pteridomonas, Dictyocha,  Pseudopedinella, Pedinella, etc

Class-4: Eustigmatophyceae

  • They are mostly small and pale green unicellular coccoid algae.
  • They inhabit in soil, freshwater and marine environments.
  • The cell is non-motile which is enclosed by cellulosic cell wall.
  • They contain one or more yellow-green chloroplasts with pigments like chlorophyll a,  violaxanthin and β-carotene.
  • The flagellate cell bears one or two flagella.
  • This class includes about 41 described species.

Examples: Eustigmatos, Botryochloropsis,Pseudocharaciopsis, Ellipsoidion , Pseudellipsoidion, Nannochloropsis,Pseudostaurastrum,   etc.

Class-5: Phaeophyceae (Brown Algae or Brown Seaweeds)

  • They are also known as brown algae or brown seaweeds.
  • They are mostly marine with unicellular or multicllular body.
  • The photosynthetic pigments are chlorophyll a and chlorophyll c.
  • They also possess accessory pigments like beta carotene, fucoxanthin, lutein, violaxanthin and diaanthin.
  • They store foods as laminarin, maninitol and oils.
  • Sexual reproduction is anisogamous, isogamous or oogamous types.
  • The male gametes possess heterokont flagella.
  • They perform alternation of generation with haplobiontic or diplobiontic life cycles.
  • This class contains about 1500 described species.

Examples: Ascophyllum, Ectocarpus, Laminaria,  Fucus, Nereocystis, Macrocystis,  Pelagophycus,  Postelsia, Pelvetia,  Sargassum, etc.

Class-6: Prymnesiophyceae (Haptophyceae)

  • The cells have typically two slightly unequal flagella.
  • The cell possesses hair like appendages known as haptonema between two flagella.
  • They are predominantly marine algae.
  • This class contains about 762 described species.
  • The exoskeleton consists of calcareous plates called coccoliths.
  • The thylakoids are stacked in triplets and it contains chlorophyll a and chlorophyll c.
  • They store food as chrysolaminarin.

Examples:  Chrysochromulina, Emiliania, Phaeocystis, Prymnesium, etc.

This class includes the following orders:

Order-1: Rappemonadales

Order-2: Pavlovales

Order-3: Discoasterales

Order-4: Phaeocystales

Order-5: Prymnesiales

Order-6: Isochrysidales

Order-7: Eiffellithales

Order-8: Stephanolithiales

Order-9: Zygodiscales

Order-10: Syracosphaerales

Order-11: Watznaueriales

Order-12: Arkhangelskiales

Order-13: Podorhabdales

Order-14: Coccolithales

Class-7: Raphidophyceae (Chloromonadophyceae)

  • They are unicellular algae with large cells which range from 50 to 100 μm.
  • They possess a pair of flagella.
  • The cell contains numerous ellipsoid chloroplasts.
  • They are photosynthetic organisms which contain pigments like chlorophylls a and c1 and c2.
  • The cells also contain accessory pigments such as β-carotene and diadinoxanthin.
  • They inhabit in both in freshwater or marine environments.
  • This class contains more than 50 described species.

Examples: Gonyostomum, Vacuolaria, Merotricha,  Chattonella, Chlorinimonas,  Haramonas, Psammamonas,Fibrocapsa, Heterosigma, and Viridilobus, etc.

This class includes the following orders:

Order-1: Commatiida

Order-2: Actinophyrida

Order-3: Raphidomonadales

Class-8: Synurophyceae

  • They are the most important photosynthetic stramenopile algae.
  • They inhabit in freshwater ecosystem.
  • They are motile organisms which contain two parallel heterokont flagella.
  • The cell is covered by siliceous scales.
  • Asexual and sexual reproduction occur. In this case, sexual reproduction is isogamous type.
  • This class contains about 200 described species.

Examples: Mallomonas, Synura

This class includes the following orders:

Order-1: Chloramoebales

Order-2: Heterogloeales

Order-3: Ochromonadales

Order-4: Synurales

Class-9: Xanthophyceae (Yellow-Green Algae)

  • They are commonly known as yellow green algae.
  • They contain yellow green chromatophores.
  • Photosynthetic pigments are chlorophyll a, chlorophyll e,  xanthophyll or carotenoids.
  • They store energy as oil and leucosin.
  • The cell wall is composed of pectic compounds.
  • They do not contain pyrenoids.
  • They perform sexual reproduction which is isogamous type.
  • The zoospores possess unequal flagella. 
  • Most of them inhabit in freshwater environments.
  • This class contains about 600 described species.

Examples: Botrydium,  Tribonema, Bumilleriopsis,  Vaucheria, etc.

Division-3: Cryptophyta

  • They are unicellular photosynthetic flagellated algae.
  • They inhabit in both freshwater and marine environments.
  • The cell is flattened shape with 10–50 μm in length.
  • The cell bears typically two unequal flagella.

Class-1: Cryptophyceae

  • They inhabit in both marine and freshwater habitats.
  • The photosynthetic pigments are chlorophyll a, chlorophyll c and phycobiliprotein.
  • The cell body is asymmetric with dorsi-ventral sides.
  • The cell possesses two anteriorly directed flagella with tubular hairs on one or both flagella.
  • They store foods as pyrenoids outside of chloroplasts.
  • The mitochondria contain flattened cristae.
  • The cell is covered with periplast with often elaborately decorated sheet or scale.
  • This class contains about 200 described species.

Examples: Cryptomonas, Chilomonas,  Falcomonas,  Rhinomonas, Plagioselmis,  Teleaulax, etc.

Division-4: Rhodophyta (Red Algae)

  • This group contains about 6,000 described species.
  • Most of them are photosynthetic marine organisms but few are parasitic.
  • Photosynthetic species contain pigments like chlorophyll a  and chlorophyll d.
  • They also contain phycocyanin carotenoids, xanthophyll, and phycobilins and phycoerythrin (phycobiliproteins) as accessory pigments.
  • The cell wall consists of cellulose and polysaccharides such as agar and carrageenin.
  • They store energy as a specialized polysaccharide, known as floridean starch outside chloroplast.
  • They do not contain flagella; mitochondria with flattened cristae.

Examples: Palmaria, Polysiphonia, Bangia, Corallina, Gelidium Chondrus,   Kappaphycus, Gracilaria,  Porphyra, Rhodymenia, etc.

Division-5: Dinoflagellata

  • They are mostly unicellular flagellated algae.
  • They are heterotrophic or autotrophic (photosynthetic) organisms.
  • Photosynthetic forms contain chlorophyll a, and chlorophyll c with accessory pigments like   peridinin or fucoxanthin.
  • The flagellum does not contain tubular hairs.
  • Mitochondria possess tubular cristae.
  • This phylum contains more than 1,500 described species. 

Class-1: Dinophyceae

  • They are also is known as dark yellow to brown algae.
  • They store food materials as starch and oil.
  • The body contains characteristics nucleus with condensed and banded chromosomes.
  • They live as free living, symbiotic or parasitic forms.
  • Often, they are commonly known as sea water planktons.
  • The motile form possesses two different flagella.

Examples: Dinophysis, Alexandrium,  Gonyaulax, Ceratium,  Noctiluca, Gymnodinium,  Polykrikos, Peridinium,  etc.

This class includes the following orders:

Order-1: Haplozoonales

Order-2: Akashiwales

Order-3: Blastodiniales

Order-4: Apodiniales

Order-5: Dinotrichales

Order-6: Phytodiniales

Order-7: Brachidiniales

Order-8: Ptychodiscales

Order-9: Amphilothales

Order-10: Actiniscales

Order-11: Gymnodiniales

Order-12: Prorocentrales

Order-13: Nannoceratopsiales

Order-14: Dinophysales

Order-15: Gonyaulacales

Order-16: Thoracosphaerales

Order-17: Peridiniales

Division-6: Euglenophyta

  • This phylum contains about 800 species. Most of them inhabit in freshwater environment.
  • They lack a true cell wall, and the body is bounded by a proteinaceous cell covering which is known as a pellicle.
  • They possess one to three flagella for locomotion.
  • They store carbohydrate as paramylon.
  • They are both photosynthetic and heterotrophic organisms.
  • The primary photosynthetic pigments are chlorophyll a and chlorophyll b.
  • They also contain carotenoids and xanthophylls as accessory pigments.  
  • They feed on organic material suspended in the water.

Class-1: Euglenophyceae

  • They are commonly known as pure green algae.
  • Most of them inhabit in freshwater environments.
  • The body is covered by  flexible pellicle which is formed of protein.
  • The reserve food materials are carbohydrates or starch.
  • The body has two definite ends such as anterior and posterior ends.
  • The body possesses large and prominent nucleus and a contractile vacuole for performing osmo-regulation.
  • Autotrophic or heterotrophic nutrition occurs.
  • The body contains two apically or laterally placed flagella which lack tubular hairs.
  • Reproduction is isogamous type.
  • They contain single lobed chloroplast with central pyrenoids. In this case, photosynthetic pigments are chlorophylls a and chlorophyll b.
  • Mitochondria contain paddle-shaped cristae.
  • This class contains about 1000 known species.

Examples: Colacium, Euglena, Eutreptiella, Phacus, etc.

Types of Algae

Based on colors, algae are divided into the following major four groups:

Blue Green Algae

They belong to the class Cyanophyceae under the phylum Cyanophyta. They inhabit in freshwater or in a wide variety of moist soils of a terrestrial environment. They also form a symbiotic relationship with plants or lichen-forming fungi. They contain pigments like chlorophyll 'a', 'b', and phycobilins and they appear in blue green color. They are also known as Cynabacteria.

Green Algae

They may be either unicellular or multicellular algae which belong to the class Chlorophyceae  under the phylum Chlorophyta. They contain pigments like chlorophyll a, chlorophyll b, carotenoids, and xanthophylls.

Examples: Chlamydomonas, Spirogyra, and Chara

Red Algae

Red algae belong to the class Rhodophyceae under the phylum Rhodophyta which is one of the largest phylum of algae. This phylum contains over 7000 recognized species. Among them, 6,793 species are found in the Class Florideophyceae. Most of the algae are multicellular and marine algae (seaweeds). Red algae contain pigments like chlorophyll a, chlorophyll d, carotenoids, xanthophylls, and phycobilins. About 5% of the red algae inhabit in freshwater environments. They are used as a stabilizer in milk products.

Examples:  Porphyra, Gracilaria, and Gelidium.

Brown Algae

These types of algae belong to the class Phaeophyceae under the phylum Phaeophyta. They contain pigments like chlorophyll a, chlorophyll c, carotenoids and fucoxanthin. They form a large group of multicellular algae. Majority of them inhabit in marine environments. Over 1500 known species of brown algae are available worldwide.

Examples:  Dictyota, Laminaria, Sargassum, etc.

Based on morphology, algae can be divided into several types. Some are filamentous forms having cells arranged in chains like strings of beads. Some filamentous are un-branched such as Spirogyra while others are branched and bushlike such as Stigeoclonium.

Algae are almost ever-present throughout the world. Ecologically, they can be grouped into the following types by their habitats.

  1. Planktonic algae: They are microscopic and grow suspended in the water.
  2. Neustonic algae: They grow on the water surface and can be either microscopic or macroscopic.
  3. Cryophilic algae: They can occur in snow and ice.
  4. Thermophilic algae: They can live in hot springs.
  5. Edaphic algae: They can inhabit on or in soil.
  6. Epizoic algae: They grow on the body of animals, such as turtles and sloths.
  7. Epiphytic algae: They can grow on other algae, fungi, and land plants.
  8. Corticolous algae: They can grow on the bark of trees.
  9. Epilithic algae: They inhabit on rock surfaces.
  10. Endolithic algae: They inhabit in porous rocks or coral.
  11. Chasmolithic algae: They grow in rock fissures.
  12. Endosymbionts algae: Some algae live inside other organisms, and this situation is known as endosymbionts. In this case, endophytic endosymbionts live in plants, fungi, or other algae.

Concluding Remarks

Algae are living organisms which are distributed throughout the world. They have different sizes, shapes, and colors. They can inhabit in freshwater and marine environments. They also grow on the body surfaces of other organisms such as turtles and polar bears, on rocks or in the soils, or under or inside porous rocks, such as limestone and sandstone.   The algae have great important because they produce much oxygen on the earth for animals and human beings. Nutritionally, they contain several healthy elements such as proteins, fats, carbohydrates, and vitamins A, B, C, and E. They also contain a number of important minerals such as iron, potassium, calcium, magnesium, zinc and manganese.

Phylum Chordata: Characteristics and Its Classification

The chordates were originated from a fish-like ancestor, very similar to the larva of the Ascidia (Tunicata) and it is assumed that they became the ancestor of the chordates by retaining the larval form throughout the life. In the palaeozoic age, the chordates were originated from some sessile Echinoderms, which were bottom dwellers of the sea. The primitive chordates resemble the non-chordates in some respects and are known as protochordates or invertebrate chordates. In the evolutionary course, They  acquired new novelties and became vertebrates that were radiated on all ecological niches and became strong rivals of the non-chordates in the course of time.

The phylum chordata is a very diverse phylum which contains about 43,000 living species. Among them, most organisms belong to the subphylum, Vertebrata. This phylum is considered as the third largest phylum in the animal kingdom.

Characteristics of Phylum Chordata

At different stages in their life, chordates show the following four features. They are:

  1. Presence of the notochord;
  2. Presence of the dorsal tubular nerve cord;
  3. Presence of the paired pharyngeal gill-slits.
  4. Presence of the Post-anal tail.

Notochord: It is an elastic, rod-like longitudinal structure that is made up of cartilage. It is situated immediately above the alimentary canal (digestive tract) and just below the dorsal tubular nerve cord. The notochord is formed of special type of vacuolated cells and remains covered by a sheath, known as notochordal sheath. In the invertebrate chordates (urochordata and cephalochordada), it is formed of endothermal cells, but in the case of the vertebrate-chordates, it persists in the adult (with some exception), but in vertebrates, notochord is either partially, or wholly replaced by vertebral column. The main function of notochord is to support nerve cord.

Dorsal tubular nerve cord: It is a single hollow nerve cord which consists of bundle of nerve fibers that exists dorsally along the antero-posterior axis of the body, just above the notochord. Embryologically, this nerve cord originates from the dorsal ectoderm.  It connects the brain to the muscles and other organs.

Paired Pharyngeal gill-slits: A pair of openings which connect the mouth and the throat, known as Pharyngeal gill-slits. At some stages of the life, all chordates possess paired pharyngeal gill-slits. In the primary aquatic animals, these remain persist in the adult and functions as the passage for the respiratory water current through the mouth, without entering the digestive system. The transition from aquatic to the land life involves pulmonary respiration and consequently the gill-slits lose their function. However, the initial stage of their formation is clearly observed in the embryos of the terrestrial chordates. In the adults, however, these are totally obliterated and become transformed into other organs like endocrine system.

Post-anal tail: It is an extension of the body away from the anus. Some chordates bear the tail with skeletal muscles, which assist in locomotion.

image of Chordates-characteristics

Image showing chordates characteristics

The phylum Chordata also contains the following characteristic features:

  • They are triploblastic animal having bilaterally symmetrical body.
  • They are coelomate organisms with organ-system level of organization.
  • They have dorsal, hollow and single nervous system.
  • They have a closed type of blood circulatory system with ventral heart.
  • They are found in various environment; some live in marine habitat, some in freshwater and others in terrestrial environments.
image of Phylum Chordata Classification

Outline Classification of Phylum Chordata

Phylum Chordata is divided into the following three subphyla:
  • Urochordata
  • Cephalaochordata
  • Vertebrata

Among the above three sub-phyla, the major subphylum is vertebrata because it contains a variety of organisms like fishes, amphibians, birds, reptiles and mammals while the first two sub-phyla, urochordata and cephalochordata are collectively known as protochoradata that have very few species in between them which are all marine animals. Besides, the phylum vertebrata bears a distinguishing backbone that is built up of bone or cartilage. They have a brain which is enclosed in a skull. They also have proper nervous system, circulatory system, and a skeletal system that provides proper shape and support.

Sub-phylum: Urochordata

The main characteristics of urochordata are:

  • They are marine animals (sessile) which are also known as tunicate and their body is enclosed in cuticular tunic or test, made up of cellulose like substance known as tunicin.
  • They mostly live at the bottom of the sea.
  • There are two openings on the body surface, mouth and atriopore.
  • Pharynx forms large sac with perforated walls which act as an organ of respiration and food capture.
  • They bear notochord only in the larval stage but in the adult stage, it disappears.
  • In the adult, nervous system is reduced to a ganglion.
  • Coelom is reduced which is restricted within pericardium.
  • They are hermaphrodite (sexes are united).
  • They show sexual or asexual reproduction. In this case, asexual reproduction occurs by budding. In a few cases, alternation of generation occurs.
  • Larva is free swimming tadpole larva which contains a notochord and dorsal hollow nerve cord in the caudal region but nerve cord is replaced by a dorsal ganglion in adults.
  • At the adult stage, they show retrogressive metamorphosis and lose their tail and notochord.
  • The adults are fixed to the substratum.
  • Larval forms are free swimming; some are solitary or form colonies.

Examples: Ascidia mentula , Salpa fusiformis, Doliolum nationalis.

The sub-phylum Urochordata contains the following three classes:

Class-1: Ascidiacea

  • Adult is sessile, unable to swim and the sac-like body remains attached to any aquatic objects.
  • Oral and atriporal aperture are close at the proximal free-end.
  • Spacious pharynx bears numerous pharyngeal gill-slits.
  • Chordate characteristics disappear in the adult but it persists in the larval stage. 

Example: Ascidia mentula

Class-2: Thaliacea

  • Adults are incapable of swimming.
  • Oral and atriporal apertures are at opposite ends.
  • Muscles are arranged in complete or incomplete circles.
  • Two large and numerous small gill-slits are present.

Example: Doliolum denticulatum

Class-3: Larvacea

  • Adult contains notochord in the tail region;
  • They are capable of swimming.
  • Minute animals without atriporal opening or atrial cavity.
  • There is only two gill-slits.
  • Gill-slits communicate directly with the external environment.

Example: Oikopleura longicaudata

Sub-phulum: Cephalochordata (Acraniata)

  • They are Fish-like marine animals with laterally fattened body devoid of paired fins, skull and limbs.
  • They possess tail throughout the life.
  • Single-layer epidermis; oral hood with cerii;
  • The notochord is found throughout life which extends forward to the anterior end of the snout, in front of the nerve cord.
  • Presence of dorsal tubular nerve cord.
  • Pharynx large, sac-like and perforated with numerous well-developed pharyngeal gill slits.
  • Blood is colorless; circulatory system without heart.
  • Ciliated nephridia serve as the excretory organs, U shaped with solenocytes.
  • Coelom well-developed and enterocoelic in development.
  • Muscles (V-shaped myotomes) and gonads are metamerically arranged.
  • Gonads without ducts.
  • Long larval life; larva asymmetrical.
  • Progressive metamorphosis occurs in their life cycle.

Examples: Mediterranean amphioxus or European lancelet:  Branchiostoma lanceolatum

image of Barnchiostoma lanceolatum

Bracnchiostoma lanceolatum

Sub-phylum: Vertebrata (Craniata)

The sub-phylum vertebrata incorporates the remarkable chordates which occupy high taxonomical ranks. All the vertebrates are placed under the single sub-phylum Vertebrata or craniata. The vertebrate possess a ‘back-bone’ which is the most characteristic features of the vertebrates. Thus, a vertebrate is defined as an animal having back bone consisting of a series of small bones (ring-like)or cartilage running throughout the length of the body along the mid-dorsal side, enclosing and protecting the spinal cord. On the other hand, all animals which do not possess vertebral column, are known as invertebrate. The vertebrates have acquired a number of characteristics features in addition to the basic chordates characterizes which are of high adaptive value and as much they are readily distinguished from the invertebrate-chordates.

Characteristics of sub phylum Vertebrata

  • Presence of endosketetal framework, and exoskeletal structures.
  • In the adult stage, the notochord is replaced by a vertebral column.
  • Presence of cranium or brain-box accompanied with a vertebral column; the vertebral column is composed of segmented cartilaginous or bony rings, known as vertebra.
  • Multi-layered epidermis is present.
  • They possess well-developed coelom.
  • They have a high degree of cephalization or distinct head.
  • The neural tube is specialized to form the complex brain; the brain is associated with special sense organs.
  • Presence of visceral arches which are variously modified in different groups vertebrates.
  • Paired lateral appendages are present in the form of limbs or fins.
  • Circulatory system consisting of a ventral thoracic heart and shows increasing structural complexities in keeping the oxygenated blood separate.
  • They have complete alimentary canal.
  • Respiratory and excretory systems are well-developed.
  • Three or four-chambered heart is present.
  • Endocrine glands are present and well-developed.

Sub Phylum Vertebrata is further classified into the following classes. They are:

  • Class-1; Cyclostomata
  • Class-2: Chondrichthyes
  • Class-3: Osteichthyes
  • Class-4: Amphibia
  • Class-5: Reptilia
  • Class-6: Aves
  • Class-7: MammaliaAmc

Class-1: Cyclostomata

  • They have long, rounded and eel-like body with scale less, glandular and smooth skin.
  • Mouth is circular in shape and suctorial with devoid of functional jaws.
  • They have no jaws; the skull is largely or completely roofed by membrane.
  • Notochord is persistent and unconstructed.
  • Skeleton is wholly cartilaginous; vertebrae incipient as sticks.
  • They have neural tube with rudimentary neural arches.
  • They have median fin but paired fins or lateral appendages are absent.
  • Fins are not supported by true fin rays.
  • Gill pouch-like and paired.
  • Absence of conus arteriosus and renal portal system.
  • Spleen and genital ducts are also absent.
  • Nasal organ is unpaired with a long sac extending beneath the brain.
  • The gonad is single without duct; gametes pass through the genital pores.
  • They inhabit in marine habitat but migrate for spawning to fresh water.
  • Development is indirect or direct.

Examples: Petromyzon marinus (Lamprey) and Myxine glutinosa (Atlantic Hagfish).

image of Petromyzon marinus

Petromynus marinus (Lamprey)

Class-2: Chondrichthyes

  • They have streamlined body with cartilaginous endoskeleton.
  • They have ventrally positioned mouth.
  • The body contains tough skin with minute placoid scales.
  • They have powerful jaws with backwardly directed teeth.
  • They have 5-7 pairs of gill slits without operculum.
  • Some species have electric organs such as Torpedo and some bear poisonous sting  such as  Trygon.
  • They posses both paired and median fins which are not supported by fin rays.
  • They have heterocercal tail; in this case, tail is unequally bifurcated.
  • Sexes are separate and in male, pelvic fin is provided with a pair of claspers.
  • Gonads are typically paired and provided with ducts that open into the cloaca.
  • Intestine bears a spiral valve.
  • Fertilization is internal, development is direct; many of them are viviparous which give birth to young baby.

Examples: Scoliodon laticaudus (Dog fish), Pristis clavata (Saw fish), Carcharodon carcharias (Great white shark),  Dasyatis thetidis (Sting ray).

image of Pristis clavata

Pristis clavata

Class-3: Osteichthyes

All bony fishes belong to the class Osteichthyes. All the representatives of Osteichthyes are cold-blooded which bear one pair of gill openings for breathing and different types of fins for swimming.  Besides these, they have bony skeleton, jaws and one pair nostrils. The class Osteichthyes is the largest group of vertebrates in the world and it contains more than 30,000 fish species which account for about 96% of all fish species. This class does not contain sharks, rays, skates, lampreys and hagfishes. They have very keen sense organs with very good eyesight. They take a wide variety food, among them, some are herbivores, some carnivores and some take any kind of food at all(omnivores).

  • They inhabit in all sort of waters such as fresh, marine, brackish-water environment.
  • They have streamlined body with terminal mouth.
  • They have bony endoskeleton. In this case, at the embryonic stage, endoskeleton is cartilaginous but it is replaced by bones in the adult stage.
  • They have usually homocercal symmetrical caudal fin.
  • The body skin is covered by different types of scales such as ctenoid,  cycloid or ganoid scales, which are dermal in origin.
  • Alimentary canal is complete with no cloaca.
  • There are four pairs of gills which are covered by an operculum on each side.
  • They have sac-like outgrowth which is known as swim bladder or air bladder. It is air filled organ arising from the dorsal wall of the oesophagus. It helps to maintain buoyancy and in some fishes such as catfish, it helps in respiration.
  • Lateral line system is well-developed which stars from the end of the operculum and ends at the base of the  caudal fin.
  • They have well developed renal portal system with mesonephric kidneys. During excretion, they release chiefly nitrogenous waste.
  • They have 2-chambered heart, one auricle and one ventricle with sinus venosus and conus arteriosus.  But in the lung fishes, the heart is three chambered with two auricles and one ventricle.
  • Sexes are separate and they usually perform external fertilization.
  • They are mostly egg laying fish (oviparous), some are ovoviviparous and their development is direct. In few cases, development is indirect with a larva, known as leptocephalus such as Anguila.

The class Osteichthyes is divided into the following two subclasses:

  • Subclass-1: Actinopterygii
  • Subclass-2: Sarcopterygii

Subclass-1: Actinopterygii

  • They are also known as ray-finned fishes due to lepidotrichia or fin rays.
  • Fin rays are attached directly to the proximal or basal skeletal elements, known as the radials.
  • It is the dominant group of vertebrates which comprises nearly 99% of the more than 30,000 fish species.
  • They inhabit throughout marine and freshwater environments.
  • The skin of the body is covered by scales like cycloid, ctenoid or ganoid. In some fishes such as silurids, scales are absent.
  • Generally, they do not have any spiracles.
  • On the top of the head, external nare is present.
  • They possess either heterocercal or homocercal type caudal or tail fins.
  • Endoskeleton is composed of either bones or cartilages.

Examples: Labeo rohita, Catla catla, Anabas testudineus, Lates calcarifer, Tenualosa ilisha, Ompok pabo, Heteropneustes fosilis

image of Labeo rohita

Labeo rohita

Subclass-2: Sarcopterygii

  • They are also known as fleshy finned or lobe-finned fish due to their lobed appearance fins.
  • Thick, pitted rhomboidal shaped ganoid scales are present on the body with cosmoid layer characteristics.
  • Heterocercal type caudal fin is found in the primitive species but diphycercal type caudal fin is present in the recent forms.
  • They have reduced type dorsal fin.
  • They perform respiration through lungs. In this case, swim bladder is replaced into lungs.
  • Basal element of pectoral fins is attached to the girdle with a branching arrangement at the tip.
  • Internal nares are present which opens into the buccal cavity through a pore, called choanae.
  • Vertebral column continues up to the tail end which divides the tail into upper and lowers lobes.
image of Latimaria

Latimeria chalumnae

Class-4: Amphibia

The term ‘amphibian’ is derived from the Greek word ‘amphíbios’ meaning both kinds of life. They are cold-blooded vertebrate tetrapods which inhabit a wide variety of habitats, including terrestrial, arboreal, fossorial, or freshwater aquatic ecosystems. There are approximately 8,100 species, of which, about 90% are frogs. Paedophryne amauensis is the world smallest amphibians, found in Papua New Guinea which grows up to 7.7 mm in length while the largest living amphibian is Andrias sligoi (South China giant salamander) which grows up to 1.8 m in length (5 ft 11 in).

General Characteristics of Class Amphibia

  • They can live in both aquatic and terrestrial habitats.
  • The body is divided into two regions such as head and trunk. In some cases, the tail may be present. 
  • The body does not contain any scales. The skin may be smooth or rough with mucus glands which make the body moist.
  • Two pairs of limbs, namely forelimbs and hind limbs with no claws on their digits (toes) which are used for locomotion.
  • They perform respiration through the lungs and moist skin. In some adults, external gills are present for respiration.
  • They have mesonephric kidneys and they release ammonia and urea as nitrogenous waste products. 
  • Fertilization is external and development is mostly indirect. Some amphibians perform internal fertilization such as Salamander and Ichthyophis (blind worm).
  • Most of the amphibians are oviparous and they lay eggs. Some are viviparous such as Salamander.
  • During their life cycle, a fish like larva occurs which is known as tadpole larva.
  • Heart is three chambered with two auricles and one ventricle.
  • The eyes have eyelids to prevent the eyes from external injury.
  • A common chamber, known as cloaca is present where alimentary canal, urinary and reproductive tracts opens which  collects discharge products before it is eliminated from the body.
  • They have well-developed brain which is attached to a dorsal nerve cord.
  • They are ectotherm organisms and they either hibernate (winter) or aestivate (summer) during extreme environmental conditions.
  • They possess a well-developed muscular system.
  • They have well-developed digestive system which is adapted to digest whole prey, swallowed by the organism.
  • They possess well-defined liver which performs several functions such as producing bile, detoxifying poisons, and storing glucose in the form of glycogen.
  • They possess a pressure releasing canal which is known as the Eustachian tube that connects the mouth cavity with the tympanic membrane.

Examples: Tailed frog  (Ascaphus truei), European fire-bellied toad (Bombina bombina),   tiger salamander (Ambystoma tigrinum), Limbless amphibians (Ichthyophis glutinosus).

image of Tiger salamander

Tiger salamander (Ambystoma tigrinum)

Class –6:  Reptilia

The word reptile comes from Latin “Reptilis’ meaning creeping. They are the creeping and burrowing vertebrates with epidermal scales. There are about 8,000 living reptiles species in the world today that inhabit all continents on Earth except Antarctica.  It includes verities group ectothermic vertebrates including lizards, snakes, crocodiles, alligators, turtles, caimans and worm-lizards, etc. They can inhabit different type habitats such as forests, deserts, freshwater wetlands and the open seas.

  • They are cold-blooded animals (poikilothermal).
  • They have two pairs of pentadactyl limbs, each with five clawed toes, often reduced or absent.
  • Skin is dry, and it is covered with horny epidermal scales often with underlying bony dermal plats; few cutaneous glands are present in skin with high levels of keratin that stops water loss through the skin.
  • Usually, they have post-anal tail.
  • They are completely lung breathers; no skin or gill respiration.
  • Heart is imperfectly four-chambered, two auricles, the ventricles being divided incompletely by a septum into two. Sinus venosus opens into right auricle, no conus and truncus arteriosus.
  • Functional kidney, metanephros with metanephric ureters.
  • Cloaca is present which gives space to receive rectum, ureters and genital ducts; cloacal opening is transverse.
  • Digestive system is complete which has a muscular opening at the base of the tail.
  • They have a well-developed brain and a central nervous system with twelve pairs of cranial nerves.
  • Foetal membranes consist of amnion, chorion and allantois.
  • Sexes are separate; fertilization is internal with direct development.
  • They are mostly oviparous, but viviparity is seen in some snakes, some of which are ovo-viviparous. 

Examples:  Green sea turtle (Chelonia mydas), Aldabran giant tortoise ( Aldabrachelys gigantea ),  Spectral pygmy chameleon (Rhampholeon spectrum), Oriental Garden lizard (Calotes versicolor), Saltwater crocodile (Crocodylus porosus), Naja naja (Cobra), Indian Krait ( Bangarus caeruleus),Horned viper ( Vipera ammodytes).

image of Green sea turtle

Green sea turtle (Chelonia mydas)

Class –6: Aves

All birds belong the class Aves, they are egg-laying endothermic vertebrate animals. There are about 10,000 living species of birds in the world that inhabit in nearly all habitats on Earth. It constitutes the largest number of species among the tetrapod classes.

  • They are  homoiothermous or warm-blooded animals and they can maintain a constant body temperature.
  • The body is covered with special integument derivatives, known as feathers, the nature`s masterpiece.
  • Their body is very light with pneumatic bones.
  • The skin is thin, loose and dry with flight muscle in the thorax; sweat gland is absent.
  • At the base of the tail, uropygeal or preen gland is present.
  • Forelimbs are modified as wings for flight while the hind limbs are covered with scales and armed with claws which are concerned with bipedal locomotion.
  • The hind limbs generally bear scales and are modified for swimming, walking, or clasping the branches of trees.
  • Horny, hard beak or bill is present.
  • Mouth does not bear teeth.
  • Head is small and round with a fairly long, flexible and movable neck.
  • Alimentary canal contains additional chambers, the crop and gizzard which are highly developed in grain-eating bird.
  • Gall-bladder is absent and rectum opens into a cloaca with tree differentiated compartments.
  • They have no urinary bladder.
  • Respiratory system can perform a double respiration. In this case, air sacs are connected to lungs which ensure supplement respiration.
  • Lung is small, elastic and is affixed in the dorsal wall of the thorax and give rise to some air-sacs to increase its efficiency.
  • The sound producing organ, known as syrinx is present
  • The heart is completely four-chambered; sinus venosus and conus arteriosus are absent.
  • Excretory organ, known as kidney is metanephric, urine is semi-solid.
  • Eyes are large in size and powerful with a specialized structure, known as pectin.
  • They are oviparous and they lay eggs with large yolk which is covered by a hard-shell.
  • Sexes are separate; fertilization is internal and their development is direct.

Examples : Corvus splendens ( house crow), Columba livia (Domestic pigeon), Indian vulture (Gyps indicus) etc.

image of Columba livia

Columba livia

Class-7:  Mammalia

The word ‘Mammalia’ is derived from Latin word ‘mamma’ meaning "breast". The class Mammalia includes the vertebrate animals which contain mammery glands. These glands are active in female and produce milk for feeding their babies. These vertebrate animals are also characterized by the presence of fur or hair, a four-chambered heart and three middle year bones which distinguish them from birds and reptiles. There are about 5500 living mammal species under 161 families and 29 orders. The rodents, bats and shrews constitute the largest order which is followed by the Primates, Cetartiodactyla and Carnivora, respectively.

  • The mammals are ectothermic (warm-blooded) animals which bear four-chambered hearts.
  • Most of the mammals give birth to live young babies except Platypus and the Echidna which lay eggs.
  • They inhabit variety of habitats such as grasslands, deserts, polar ice caps, forests, mountains, dark caves and even some of them can adapt to fly or live in water.
  • They have two pairs of limbs which are used for walking, climbing, running, burrowing, flying or swimming.
  • The skin of mammals is unique which possesses hair and oil glands such as sebaceous glands and sweat glands. In this case, hair or fur helps animals adapt to their environment.
  • They have well-developed brain which is divided into cerebrum, cerebellum and medulla.
  • They perform respiration through lungs.
  • They have 12 pairs of cranial nerves and exhibit one of the most highly developed forms of Diaphragms
  • Sexes are separate and they perform internal fertilization.
  • Most of them are viviparous with few exception and they show direct development.

Examples: Oviparous – Platypus (Ornithorhynchus anatinus); Viviparous –  Royal Bengal Tiger (Panthera tigris),Red kangaroo (Macropus rufus), flying fox (Pteropus vampyrus), Short-beaked common dolphin (Delphinus delphis) ,  blue whale (Balaenoptera musculus) , etc.

image of Royal Bengal tiger

Royal Bengal tiger (Panthera tigris)

Difference  Between Chordates and Non-chordates

Chordates

Non-chordates

They are cold or worm blooded organisms.

They are cold blooded organisms.

Notochord is present at some stage or it is replaced by a ring like vertebrae at adult stage.

Notochord or backbone is absent.

They have central nervous system which is dorsal, hollow and single.

In this case, central nervous system is ventral, solid and double.

Pharynx is perforated by gill-slits.

In this case, gill-slits are absent.

They perform respiration through gills or lungs.

They perform respiration through body surface, gills,or tracheae.

The position of heart is ventral.

Usually, the heart is absent, if present the position of heart is dorsal or lateral.

A post-anal part or tail is present.

In this case, post-anal tail is absent.

They are true coelomate organisms.

They may be acoelmate, pseudocoelmate or truly coelmate organisms.

Blood vascular system is closed type.

Generally, blood vascular system is absent;  if present, it may be open or closed type.

Their body is bilaterally symmetrical.

Their body is  radial, bi radial, bilateral or without symmetry.

Nerve cord is single, dorsal, without ganglia.

Nerve cord is double, ventral, usually with ganglia.

The gut is situated ventral position to nerve cord.

In this case, gut is found dorsal position to nerve cord.

They mainly perform sexual reproduction.Enter your text here...

In this case, asexual reproduction is predominant.

Their regeneration power is usually poor.

Their regeneration power is usually good.

They possess both exoskeleton and endoskeleton

In this case, only exoskeleton is present.

Examples: Hemichordates,  Cyclostomes, Aves, Reptiles, Amphibians, Mammals , etc.

Examples: Protozoans, Annelids, Arthropods, Echinoderms, etc

Concluding Remarks

Chordates are animals which belong to the phylum Chordata. This phylum consists of three groups, namely vertebrates, tunicates, and lancelets. The members of chordates are very essential for an ecosystem because they keep up the ecosystem. Chordates consist of 43,000 living species which can consume and hunt down other types of animals and maintain the of predator and prey role. Many chordates provide food for human beings and many chordates release their excretion to the environment, provide nutrients and minerals to the ecosystem which boost up the plants growth.

Phylum Platyhelminthes : General Characteristics and Its Classification

The representatives of the phylum Platyhelminthes are commonly known as the flatworms or tapeworms. The word ‘Platyhelminthes’ is derived from the Greek word, ‘platy’ meaning flat and ‘helminth’ meaning worm. They are simple soft-bodied, bilaterian, unsegmented invertebrate animals. The Phylum Platyhelminthes makes up the 4th largest phylum among the animal kingdom. But among the acoelomate organisms, the phylum Platyhelminthes constitutes the largest phylum with more than about 20,000 known species throughout the world. Among them, around 80% live as parasitic life on humans and other animals and few are free-living. 

The parasitic forms cause some trouble to the host animals, feed on host`s tissues and make certain diseases such as Schistosomiasis, or snail fever, Taeniasis, etc. while the free-living flatworms are scavengers or predators. Generally, free-living species live in water and some in shaded, humid terrestrial ecosystems, such as leaf litter. The members of this phylum have diverse sizes which range from microscope to 3 feet long.

General Characteristics of Platyhelminthes

  • They have dorso-ventrally flattened the un-segmented and tape-like or leaf-like body.
  • They live free-living or parasitic life.
  • The body is soft and does not contain any cilia.
  • They have a bilaterally symmetrical body with triploblastic and acoelomate.
  • They have an organ-system grade of organization and cephalization.
  • The digestive system is incomplete or absent with no anus. In this case, the true stomach is absent and pharynx opens into a complex intestinal structure.
  • They perform excretion through flame cells, protonephridia or solenocytes. In this case, they release fatty acids, ammonia, and CO2 as excretory products.
  • Respiration and nutrition occur in their body through the general body surface.
  • The nervous system consists of anteriorly situated ganglia and nerve cords which run along the body.
  • They are monoecious or hermaphroditic organisms and have well-developed reproductive organs.
  • Internal fertilization occurs and direct development occurs in free-living forms and indirect in parasitic forms.
  • During reproduction, they produce eggs with yolk which is covered by the shell.
  • During embryogenesis, spiral cleavage occurs and they show complete life cycle with many larval stages.

Classification of Platyhelminthes

The Phylum Platyhelminthes is classified into the following classes:

  • Class-1: Turbellaria
  • Class-2: Monogenea
  • Class-3: Cestoda
  • Class-4: Trematoda

Class-1: Turbellaria (Latin, turbella = a string)

Class Turbellaria contains about 3,000 species, of which, majority of these species live in marine environments, some are found in freshwater environments and few live in tropical terrestrial and moist temperate environments.

  • They have an elongated and relatively soft body with tapered both ends.
  • The body is dorso-ventrally flattened with no hooks and suckers.
  • They are acoelomate and their body does not contain any segmentation.
  • They do not have an anal opening. In this case, food is taken through the pharynx and expelled through the mouth.
  • The majority of the species of this class are predators of smaller invertebrates while some others live as herbivores, scavengers, and ectoparasites.
  • They show sexual and asexual methods of reproduction. In this case, sexual reproduction occurs through producing eggs and cocoons while asexual reproduction through the regeneration process.

Examples: Pseudobiceros bedfordi, Pseudoceros dimidiatus

image of Pseudobiceros bedfordi

Pseudobiceros bedford

Class Turbellaria includes the following orders:

  • Order: Acoela
  • Order: Neorhabdocoela
  • Order: Catenulida
  • Order: Macrostomida
  • Order: Tricladida
  • Order: Proseriata
  • Order: Polycladida
  • Order: Lecithoepitheliata
  • Order: Kalyptorhynchia

Class-2: Monogenea

It is one of the largest groups of flatworms and most of the members live in the aquatic environment and they lead exclusively parasitic life (ectoparasites) of aquatic vertebrates.

  • They have flattened cylindrical and leaf-shaped body with oral suckers.
  • The body bears a large posterior adhesive disk which is known as opisthaptor. It helps to attach to the body of the host. By using these organs, they feed off the outer epidermal layer of the body of the host.
  • The head is located at the anterior region which contains eyespots with pigments.
  • The anterior region also contains poorly developed oval-shaped pharynx.
  • The body does not contain an anal opening. To eliminate waste product, they use the protonephridia system.
  • The respiratory and circulatory systems are absent.
  • The nervous system consists of a nerve ring and nerves.
  • They are hermaphrodites and fertilization is external with free-swimming larva.

Example: Diplozoon paradoxum, Gyrodactylus adspersi

image of Diplozoon paradoxum

Diplozoon paradoxum

Class-3: Cestoda (Greek, kestos = girdle, eidos = form)

  • The member of the class is commonly known as tapeworms.
  • They contain over 4,000 species which lead endoparasitic life forms.
  • They have long flat and tape-like bodies which can grow up to 18 meters long.
  • They have no digestive, circulatory and respiratory systems.
  • The body bears large numbers of male and female reproductive structures which are known as proglottid that are capable of reproducing independently.
  • They pick up nutrition using saprozoic method due to their lack of digestive system. In this case, their body surface is covered by small microvillus-like projections which effectively absorb nutrients.
  • They can capable of producing thousands of eggs which hatch to produce larvae, known as coracidium.
  • The body bears well-developed muscle.
  • The body surface also contains modified cilia which are used as sensory endings.
  • The nervous system consists of a pair of lateral nerve cords.
Class Cestoda is divided into the following two subclasses:

Subclass-1: Cestodaria

  • This subclass contains about 15 species.
  • They live endoparasitic life and can be found in the intestine of primitive fish.
  • The body is unsegmented with no scolex.
  • They have a single set of the reproductive organ which may be either male or female.
  • The body contains a sucker or adhesive organ which is located on the posterior side.
  • They do not have a digestive system and parenchymal muscle cells.

Example: Gyrocotyle rugosa, Amphilina foliacea

image of Amphilina foliacea

Amphilina foliacea

The subclass Cestodaria consists of the following orders:
  • Order: Amphilinidea
  • Order: Caryophyllidea, and
  • Order: Gyrocotylidea

Subclass-2: Eucestoda

  • This class contains over 3,000 species.
  • The members of this subclass are known as true tapeworm.
  • They have an elongated and white-opaque dorso-ventrally flattened body with few mm to 25 m in length.
  • They have no mouth or digestive tract. In this case, they absorb nutrients through the body wall surface.
  • The body is divided into scolex, neck, and strobila that contain a series of units, known as proglottids. In this case, proglottids play an important role in reproduction.
  • They lead intestinal parasitic life with many sets of reproductive organs.
  • The larval stage is known as hexacanth which bears 6 posterior hooks.

Example: Taenia solium, Diphyllobothrium latum

image of Diphyllobothrium latum

Diphyllobothrium latum

Class-4:Trematoda (Greek, trema = hole, eidos = form)

The members of this class are commonly known as flukes. There are more than about 20,000 known species of class Trematoda which are all parasitic in nature.

  • Their alimentary canal and excretory system are well developed. In this case, the alimentary canal is highly branched with no anus where mouth as the only opening.
  • They bear a well developed muscular system.
  • Sexes are separate with the complex reproductive system. Their life cycle involves two types of the host such as intermediate and main host.
  • They can live in blood for several years. In this case, adult flukes feed on the blood of the host.
  • This class is characterized by a complex hermaphroditic reproductive system and a life cycle that involves intermediate and main hosts.
  • The body bears oral and ventral suckers by which they make them attach with the host body for feeding easier.
The class Trematoda is divided into the following two subclasses:

Subclass-1: Aspidogastrea

  • This Subclass Aspidogastrea contains about 80 species under 4 families.
  • They live as endoparasites of both marine and freshwater mollusks, fishes and reptiles.
  • They have a complex nervous system.
  • The entire ventral surface is covered by suckers with many small alveoli.
  • Small microtubercles are present in the tegument.
  • The gut bears a single caecum.

Example: Multicotyle purvisi, Lobatostoma manteri

Subclass-2: Digenea (Greek, Dis – double, Genos – race) 

  • This subclass consists of over 18,000 species, under 80 families.
  • Their life cycle is complex that requires several hosts such as intermediate hosts (mollusks) and definite host (vertebrates).
  • The larval stage is known as miracidium.
  • They have flattened leaf-like or ribbon-like body.
  • They occur throughout the world and their body size ranges from 5 mm to 10 cm in length.
  • The ventral surface of the body bears suckers.
  • The body also bears hooks and spines which are used for attachment to the body of hosts.

Example: Lecithochirium sp.,  Pycnoporus heteroporus

image of Lecithochirium sp

Lecithochirium sp

Subclass Digenea includes several orders that include:
  • Order: Strigeidida
  • Order: Echinostomida
  • Order: Plagiorchida
  • Order: Opisthorchiida

Concluding Remarks

The members of the phylum Platyhelminthes are known as flatworms which can adapt to an enormous variety of habitats. Flatworms live as endoparasites in the intestines and digestive tracts of the human body. Several species such as Cestodes (tapeworms) and digeneans (flukes) can cause diseases in livestock and human beings. In many countries, serious losses of stocks in fish farms are occurred by the monogeneans. The genus Schistosoma bores skin of human and causes diseases, known as Schistosomiasis, or bilharzia or snail fever. Besides these, many species of Platyhelminthes play an important role for healthy streams, lakes, and ponds. They also offer food for animals such as dragonflies, when they are young.

Pinus: Salient Features, Morphology and Reproduction

Pinus are coniferous, evergreen resinous trees which are popularly known as pine. They belong to the family Pinaceae under order Coniferales of class Coiniferosida. Different species of the genus Pinus are distributed throughout the temperate and sub-alpine regions of Northern Hemisphere where they form dense forests of evergreen trees. They can grow up to 80 (260 ft) meters in height. The tallest pine tree is ponderosa pine which can grow about 81.79 m (268.35 ft) tall. They are long lived plant and can live up to 1000 years or more under favorable conditions.

There are about 120 species of pine tree throughout the world. They are widely distributed in the hills and have economic value because many pines are used in the construction, paper-products industries as the source of timber and resin. Some pine trees are also sources of wood tars, resin, turpentine, and oils while some produce edible seeds such as pine nuts, piñons, or pinyons. Besides these, white, black, Himalayan, and stone pines are cultivated as ornamental pine trees.

Systematic Position

Division: Coniferophyta

Class: Coiniferosida

Order: Coniferales

Family: Pinaceae

Genus: Pinus

Species: P. wallichiana, P. insularis, P. armandi, Pinus khasya, Pinus geradiana, Pinus roxburghii, etc.

Salient Features of Pinus

  • They are evergreen, perennial lofty trees with spirally growing branches which give pyramidal or conical appearance.
  • The body is divided into stem, roots and needle-like leaves.
  • The stem is erect and cylindrical and is cov¬ered with bark.
  • There are two types of branches: the long shoot of unlimited growth and dwarf shoot of limited growth.
  • The long shoot bears apical bud and grows indefinitely with many scaly leaves. 
  • Dwarf shoot does not contain any apical bud and they arise on the long shoot in the axil of scaly leaves.
  • Each dwarf shoot bear two scaly leaves which is also known as  prophylls.
  • Leaves are dimorphic: the long green needle shaped foliage leaves and small, brown, membranous scale leaves.
  • Scale leaves are thin and brownish in color which is developed only on long as well as dwarf shoots while the foliage leaves are large, needle-like and found only at the apex of the dwarf shoots.
  • The pine bears tap root system with insufficient hairs but it disappears soon. Many lateral roots also develop which play an important role to absorb the mineral containing water. 
  • The branch roots are infested with mycorrhizal fungus and hence it is called the mycorrhizal root.
  • They have endarch vascular bundles. Individual vascular bundles are separated by means of medullary rays.
  • The anatomy of leaves shows xerophytic structure: thick cuticularised epidermis with sunken stomata and sclerenchymatous hypodermis.
  • Resin ducts are present in the mesophyll tissue and the cells of the mesophyll have ridges on the walls which project inside the cell cavities.
  • Microsporophylls are arranged spirally on the central axis and forms male cone.
  • Megasporophyll of the female cone is composed of large ovuliferous scale and lower smaller bract scale, which are the free from each other.
  • Each ovuliferous scale bears two anatropous ovules or megasporangia.
  • The pollen grains are winged.
  • During the development of male gametophyte, two prothalial cells are formed which later on degenerates. Besides these, 2-3 archaegonia are formed with a neck of eight cells.
imaGE OF MATURED PINE TREE

A Mature Pine tree (Pinus resinosa)

External morphology of Pinus

The plant body represents the sporophyte. The sporophytes are evergreen and tall tree (10 -80 m in height). The body has three parts: root, stem and leaves. The plants bear well developed tap root system. The stem is stout, branched and pyramidal in shape with recemose branches.

Root: A strong tap root system is present in young plant which may persist or roots develop and become stronger adventitious roots with increasing age.  The roots can grow on rocks or hard ground and spread over a large area. The lateral roots are well developed with insufficient root hairs. Often a branch roots are infested with mycorrhizal fungus and hence it is called the mycorrhizal root.

Stem: The stem is erect, stout, cylindrical and pyramidal shape with dimorphic branches. The branches are restricted in the apical region. The stem is covered with bark. There are two types of branches:

The long shoot of unlimited growth: The main branches or long shoots have an unlimited growth with scale leaves which are found below the dwarf shoots and the needle like foliage leaves are present exclusively at their terminal ends.

The dwarf shoot of limited growth: The dwarf shoots develop in the axils of scale leaves on the main branches, which are without apical buds. It is about 1 -2 cm long with one or two scale leaves. The dwarf shoot also contains foliage leaves. In this case, a dwarf shoot with its foliage leaves is known as spur.

Leaf: The pine tree bears two types of leaves:

The scale leaves: Both long and dwarf shoots bear scale-leaves and fall off as the branches attain maturity. These leaves are small, brownish in color and membranous with protective structures.

The foliage leaves: The dwarf shoots bear foliage leaves.  The leaves  are long, green, simple, needle-like with photosynthetic structures. They develop in clusters at the apex of the dwarf shoots and can form the spur. Their number varies from 1-5 in different species.

image of Shoots and leaves of Pinus

Internal Morphology

Stem: A young stem in cross section resembles a dicotyledonous stem in many respects. It is wavy in outline. The general arrangement of various tissues from the periphery to the center is as follows:

Epidermis: It is the outer surface layer of the stem. It consists of a single layer of tubular close compact parenchymatous cells with a thick cuticle.

Hypodermis: Below the epidermis, a hypodermis layer is preset. It consists of a few layers of lignified sclerenchymatous cells.

Cortex: It consists of several layers of parenchymatous cells with copious resin ducts. A single layer of resin secreting glandular epithelial cells surrounds the resin duct.

Endodermis: Outside of the pericycle, endodermis is present. It consists of a single layer of parenchymatous cells.

Pericycle: It consists of several layers of parenchymatous cells which are located outer to the ring of vascular bundles.

Vascular bundles: They are conjoint, collateral, open, arranged in ring and endarch. In this case, protoxylem is directed towards the pith. Between xylem and phloem, a narrow strip of cambium is present. The xylem is composed of tracheids with bordered pits and xylem rays, without vessels. The phloem is composed of sieve tubes and phloem parenchyma without companion cells.

Pith: It is composed of a mass of parenchymatous cells.

Pith rays: It is composed by a narrow strip of cells running from the pith outwards between the vascular bundles.

image of Stem of Pinus (Cross section)

Stem of Pinus (Cross section)

Leaf: A transverse section of foliage leaf shows the following structure under the microscope. It is semi-circular in outline (centric type of leaf).  

(i) Epidermis: It consists of a single layer of thick walled cells with a heavy sheet of cuticle and sunken stomata.

(ii) Hypodermis: It consists of two or three layers of very thick walled sclerenchymatous cells with resin ducts, which are broken up by stomata. It is placed just underneath the epidermis. It strengthens the tissue of the leaf.

(iii) Mesophyll: It consists of a few layer of chloroplast containing parenchyma cells with peg-like projections. In this case, wall projecting occurs inside the cell cavity which is known as arm palisade.

(iv) Endodermis: It consists of a single layer of barrel shaped cells which is present outside of the pericycle.

(v) Vascular bundles: They are collateral, closed and two in number.

(vi) Pericycle: It consists of albuminous cells and tracheidal cells. In this case, albuminous cells lie close to the phloem while tracheidal cells lie adjacent to the xylem. Albuminous and tracheidal cells together form the transfusion tissue which helps to flow of nutrients. The xylem is surrounded by pericycle.

image of Needle leaves (cross section)

Needle leaves (cross section)

Root: The internal organization of tissues in the root of Pinus is almost similar to that in a dicotyledonous root. A cross section of young root shows an epiblema (piliferous layer) with root hairs, a multilayered cortex and a diarch to pentrarch vascular cylinder and Y-shaped xylem bundles. 2-3 layered pericycles surround the vascular bundles while the cortex consists of a few layers of thin walled parenchymatous cells. A single layered endodermis is present which is followed by a pericycle. The root is mycorrhizal because fungus grows on the surface of the root.

Reproductive Structures of Pinus

The Pinus is monoecious plant which shows the sporophytic generation. The microsporophyll (male) and megasporophyll (female) are formed on the same plant but these two types of sporophylls appear usually in separate cones or strobilli.

The male and female cones are known as staminate strobilus and carpellate strobilus, respectively. The Pinus does not show vegetative reproduction. The flowers are unisexual. They always occur on the shoots of the current and a little away from the apex.

Male cone (staminate strobilus)

The male cones are simple, compact, oval structures and about 2-3 cm long. They are found to occur in clusters, near the tip of the long shoots.

Each male cone bears a short and elongated central axis upon which a large number of microsporophylls or stamens are arranged spirally. The microsporophylls are scaly and their number varies from 60 to 135 in each cone.

A microsporophyll consists of a short filament or stalk and a terminal leaf-like expanded structure while the apex is slightly bent upwards. Each microsporophyll bears two pouch-like microsporangia (anthers or pollen sacs) on its ventral surface. A microsporangium is sessile and oblong which is supported with a jacket of several layers of cells.

Each microsporangium produces a several microspores (pollen grains). The wall of each microspore is covered by inner intine and an outer exine.  The microspores are winged and yellow in color. In this case, wings help in the dispersal of spores by wind.

image of Male cones of Pinus pinaste

Male cones of Pinis pinaster

image of Male-cone of Pinus

Female Cone (ovulate strobilus)

The female cones are larger and compound in nature. They are formed in clusters of 1-4 in the axils of scale leaves of long shoots. Initially, they are green but ultimately become brownish red in color. It starts to produce in winter and become ready for pollination during the spring. It is hard woody and dry structure. It bears a central axis upon which a large number of megasporophylls are arranged spirally.

Each megasporophyll has short stalk with a large ovuliferous scale on the upper surface and a small bract scale on the lower surface. Each ovuliferous scale bears two inverted megasporangia on its upper surface towards the base.

Each megasporangium consists of a massive tissue which is called the nucellus and an envelope which is known as the integument. At the basal region, the integument is fused with the nucellus and open at the top by forming micropyle.

A single megaspore mother cell is differentiated within the nuceller tissue, which divides meiotically to form four megaspores. Of these four megaspores, only the lower most one is functional while others degenerate. The only functional megaspore increases in size and takes part in the development of the female gametophyte. 

image of Female cones and foliage of Pinus contorta

Female cones and foliage of Pinus contorta 

image of Female-cone of pinus

Structure of the gametophytes

Male gametophyte: The microspore is the first cell of the male gametophyte. Microspore begins to germinate within the microsporangium and produces an extremely reduced male gametophyte. The microspore first divides to form a very small first prothellial cell and a large cell. The large cell then cuts off a second prothellial cell adjacent to the first one and the remaining of the large cell forms the antheridial cell. The two prothellial cells soon degenerate. The antherdial cell again divides to form a small generative cell above and a large tube cell below. The nucleus of tube cell is known as tube nucleus which regulates the growth of the pollen tube.

The microspores are dispersed with the help of wind. Further development of microspores takes place when it reaches the ovule due to pollination. A number of microspores reach the megasporangium (ovule) where they are attached with sticky mucillagenous substance from the micropyle.

image of Ovule of pinus

Ovule of Pinus

When mucillagenous substance dries up, some of the microspores are drawn inside on to the apex of the nucellus. Later on, the spore coats split between the wings. Then the tube cell protrudes and grows to form the pollen tube. Ultimately, the pollen tube penetrates the nucellus. The generative cell then divides to form a sterile stalk cell and a fertile body cell (spermatogenous cell). The body cell further divides to form two non-motile unequal male gametes (sperms).

Female gametophyte: The functional megaspore is the first cell of the female gametophyte. It germinates within the megasporangium. The functional megaspore enlarges in size, its nucleus divides repeatedly by free nuclear divisions to form about 2,500 daughter nuclei. All the nuclei lie in the cytoplasm of the megaspore.

After that, a large central vacuole is produced, whereby the cytoplasm together with the nuclei moves towards the periphery. After pollination, development of the female gametophyte takes place again.

The walls are broken down around the nuclei and ultimately, a solid mass of thin walled cells are formed. The massive tissue thus formed within the megaspore. It is known as the female gematophyte. It is often referred to as the endosperm. The endosperm tissue is haploid (n).

A few flask shaped archegonia (2-3) are formed from the superficial cells (archegonial initials) that lie towards the micropylar end of the female gematophyte.

A mature archegonium consists of a neck of eight cells, one ventral canal cell and a large egg. There is no neck canal cell.

Fertilization

After a year of pollination, fertilization occurs. The pollen tube moves downwardly and reaches the neck of the archegonium. It then penetrates the neck and the tip of which bursts to discharge the two male gametes. One of the male gamete unites with the egg to form a diploid zygote.

New Sporophyte

The zygote is the first cell of the sporophyte which germinates almost immediately after its formation. The zygote nucleus divides to form four free nuclei. These nuclei then moves to the bottom of the zygote where they divide again to form eight nuclei. Subsequently, walls appear between these nuclei except the upper two. Further divisions result in the formation of sixteen cells in four tiers; this sixteen celled structure is termed as the proembryo.

The upper most tiers of four open cells merges into the general mass of the cytoplasm to carry out nutritive function. The next tier of four cells is called rosette tier which is capable of forming abortive embryo; normally it supplies nutrients to the suspensor and the embryo.

The third tier of four cells constitutes the suspension tier. The lower most tier of four cells is the embryo tier. The cells of the suspension tier elongate to form four separate suspensors, each carrying an embryo cell at its tip. The cells of the embryo tier form the embryos. The elongated suspensors push down the embryos into the gametophytic tissue and thus help the later to receive their nutrients.

image of Different stages of embryogeny

Each cell of the embryo tier divides to form four potential embryos and secondary suspensor. Formation of more than one embryo in each megasporangium is referred to as polyembryony. In fact, only one of these embryos attains maturity and the others undergo degeneration.

After fertilization, the integument and the megasporangium are converted into seed coat and seed respectively. The seed contains embryo, the membranous nutritive layer perisperm and kernel. The nutritive tissue lies surrounding the embryo. A mature embryo consists of a radical, a hypocotyls, several cotyledons and a plumule. The seed germinates under favorable conditions to form a new sporophyte. This type of germination is called epigeal germination.

Final Words

Pine trees are widely distributed throughout the world and have lots of economic value. Pine tree wood is very strong and it is extensively used to manufacture doors, electric poles, window panes, boats, railway sleepers, musical instruments, boards, boxes, veneers and plywood, building construction, paneling, etc. due to its durability. Turpentine oil is produced from the pine tree which is used as a solvent for varnishes, paints and in perfumery industry. It is also used for producing disinfectants, synthetic pine oil, denaturants, and insecticides, etc. Pine oil from the pinewood is used in pharmaceutical industries, textile industries, leather industries, etc.

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