Phylum Platyhelminthes : General Characteristics and 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

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

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

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

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, fishesand 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

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.

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