Phylum Annelida: General Characteristics and Its Classification
The representatives of the phylum Annelida are known as segmented worms or ringed worms. Annelida is derived from Latin word ‘anellus’ meaning little ring. It is a large phylum with over 22,000 living species. Among them, Polychaetes constitute about 12,000 species while Clitellates contains about 10,000 species. The major groups include earthworms, ragworms and leeches. The size of the body varies from microscope to 3 miters. The largest annelids are Australian giant Gippsland earthworm (Megascolides australis) with 300 to 400 body segments which can grow up to 3 meters.
General characteristics of Phylum Annelida
They have homonomous metamerically segmented body.
They are mostly aquatic but some are terrestrial.
They have red colored body due to the presence of haemoglobin in blood.
The body has three regions such as prostomium, trunk and pygidium. In this case, prostomium contains distinct head which bears tentacles, eyes etc.
Trunk region contains a longitudinal series of similar segments.
They are triploblastic organisms with true coelom. In this case, coelom is of schizocoelic origin which is divided segmentally into compartments by septa.
The body wall is covered by fibrous collagenous cuticle which supports epidermis, dermis, musculature and parietal peritoneum relatively.
They have chitinous bristles, also known as setae which project out from the epidermis that helps the body to grab on the substratum during locomotion.
They have complete digestive system. In this case, alimentary canal is straight and muscular type with digestive glands which occurs in the walls of the alimentary canal.
Respiration occurs through the body wall and gills (diffusion process). In many cases, parapodia are modified to form gills for respiration.
They have closed type blood vascular system. In this case, blood often contains a respiratory pigment.
Metanephridia are the excretory organs which are arranged segmentally and open into coelom by nephrostome.
Nervous system is well-developed which consists of a dorsal, bilobed brain and a pair of connective nerves. The nerves encircle the digestive tract and collectively to form a ganglionated double ventral nerve cord with one ganglion per segment.
They have sensory structures which include eyes and simple receptors like photoreceptors, chemoreceptors and mechanoreceptors.
Fertilization may be internal or external.
Development may be direct or indirect. When sexes are separate, they show direct development with no larval stage. When sexes are united, they show indirect development with a larval stage (Trochophore larva).
On the basis of setae, parapodia, metameres and other morphological traits, phylum Annelida is classified into following three main classes:
Class-1: Polychaeta
Class-2: Oligochaeta
Class-3: Hirudinea
Class-I: Polychaeta (Gr.polys=many, chaite=hair)
This class contains more than 6,000 living species.
They are free-moving and sedentary tube-dwelling organisms.
The representatives of this class are commonly known as bristle worms.
They are mostly marine organisms but a few live in fresh water.
Body is segmented which contains lateral paired appendages or parapodia for locomotion. Parapodium bears numerous long setae as bundles that help in fixation.
Head is distinct which consists of two pairs of eyes, palps and tentacles.
Body is bilaterally symmetry with true coeloms.
Digestive system is complete and circulatory system is closed type.
They have well-developed nervous system while excretory system consists of both metanephridia and protonephridia.
The body does not have clitellum.
The sexes are separate and fertilization is external.
Development shows larval stage; larva is free-swimming, lknown as trochophore larva.
Gonads are present in most of the body segments but some species do not contain gonads.
Examples: Neanthes succinea ( the sandworm), Chaetopterus variopedatus (the paddle worm) etc
Neanthes succinia
Chaetopterus variopedatus
Order-1: Aphroditamorpha
They are commonly known as scale worms.
They are widely distributed free-moving and segmented marine worms.
Head bears 1 or 3 tentacles, 2 palpi and 4 tentacular cirri used for feeding and respiration.
The size of the worm ranges from 0.5 to 25 cm (0.2 to 10 inches).
The back is somewhat arched with flattened pairs of overlapping scales.
They have extended proboscis or snout.
They have soft, finger-like projections and four horny jaws
Examples:Aphrodita aculeata (sea mouse), Halosydna galapaginas (common scale worm), etc.
Aphrodita aculeata (sea mouse)
Order-2: Amphinomida
They are free moving organisms.
Prostomium bears 1-5 antennae, 2 palpi and a caruncle.
Parapodia have two lobes and branchiae (gills).
The size of body ranges from 0.5 to 35 cm.
Example:Eurythoe complanata (fireworm)
Eurythoe complanata (fireworm)
Order-3: Spintherida
They have small oval body with median antenna on prostomium.
Pharynx is retractable in nature.
Dorsal side bears membranous ridges while ventral setae are strongly curved.
They are found on sponges with single genus Spinther.
Example:Spinther alaskensis
Order-4: Phyllodocida
They are mostly marine but some are found in brackish water
They are free moving organisms with a protrusible proboscis.
Prostomium bears 1 to 5 antennae, with palpi, and with 0 to 3 pairs of eyes.
Body is segmented and each segment bears a pair of paddle-like parapodia.
Parapodia are well developed into 1 or 2 lobes with usually compound setae.
The male gonopores are located in segment immediately behind testes.
Seminal receptacle is situated at or near segment containing testes.
The body length ranges from minute 3 cm.
Examples: Nais elinguis
Nais elinguis
Class-3: Hirudinea (L. Hirudo=leech)
This class contains about 300 living species.
They are primarily live in freshwater, few in sea and other on moist land.
They are haemocoelomate with open type of circulatory system.
The body lacks a head and setae and bears definite number of segments (33 in number).
The body does not contain parapodia, tentacles,and setae.
Body bears small sucker at the anterior end and large sucker at the posterior end. In this case, both suckers are used in locomotion and for feeding blood.
They are blood sucking ecto-parasites whose coelom is reduced due to the presence of nutrient-storing botryoidal tissue which is excretory in nature.
Gonads with gonoducts are present in some genital segments.
They are copulating hermaphroditic organisms (Sexes are separate) and they lay fertilized eggs laid in a cocoon secreted by clitellum.
Body bears temporary clitellum which develops during the breeding season and is located in 11th to 19th segments.
They have complete alimentary canal with many crop and caeca.
Fertilization is internal, development is direct with no larval stages.
The annelids are triploblastic invertebrate organisms with true coelom and bilaterally symmetrical body. They can survive in various ecological conditions, some live in marine environments, others live in fresh water, and yet others can live in moist terrestrial environments. Their body coelom is filled with coelomic fluid which acts as hydrostatic skeleton. The ceolomic fluid also helps for burrowing and locomotion.
Some burrower annelids such as earthworms loosen the soil and help to mix the soil with organic and mineral matter and make the soil more fertile. Earthworms also offer food for many birds such as robins, stroke, mammals like shrews, badgers, etc. Some annelids like leeches are harm to other animals due to their blood-sucking behavior. Besides these, Hirudo medicinalis is used to help in microsurgery. In this case, their saliva provides anti-inflammatory compounds and anticoagulants which prevent tumors from spreading.
