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In this article we will discuss about the General Characters and Classification of Phylum Platyhelminthes.
General Characteristics of Phylum Platyhelminthes:
(Gk. platys- broad or flat; helminworm)
Gegenbaur (1859) placed flat worms in a separate group and gave the term Platyhelminthes. The phylum Platyhelminthes includes about 13,000 species.
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General Characters:
1. Habitat:
The flat worms arc mostly parasites but some flat worms are free-living (terrestrial, fresh water or marine).
2. Body form:
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The body is dorsoventrally flattened and is without true segments.
3. Symmetry:
They show bilateral symmetry.
4. Germ Layers:
From the evolution point of view, they are the first triploblastic animals, viz., consisting of three germ layers: ectoderm, mesoderm and endoderm.
5. Level of Organization:
The flatworms have organ-system level of organisation.
6. Body wall:
The body covering is soft and may or may not be ciliated. Rod shaped bodies, the rhabdites are present in the epidermal cells of the living Platyhelminthes. They are protective and used in food capture.
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7. Body cavity:
The space between the body wall and organs is filled with a peculiar connective tissue, called the parenchyma. The parenchyma helps in transporting the food materials. Flat worms are acoelomates (without coelom).
8. Digestive Tract:
The digestive tract, if present, is incomplete (without anus). Digestive tract is absent in tapeworms.
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9. Skeletal, respiratory and circulatory systems are absent:
The fluid in the parenchymal network maintains the body shape. It is called hydro skeleton. Gaseous exchange in aerobic flat worms occurs by diffusion through body surface.
10. Excretory system:
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It consists of peculiar flame cells (solenocytes/protonephridia) which are meant for excretion and osmoregulation. Ammonia is chief excretory waste.
11. Nervous System:
The nervous system is ladder-like. It consists of the brain and two main longitudinal nerve cords connected at intervals by transverse commissures.
12. Reproductive System:
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These animals are generally hermaphrodite, and the reproductive organs are well developed. The fertilization is cross and internal. In tapeworms self fertilization is found. Asexual reproduction by transverse binary fission occurs in some flat worms.
13. Development:
Life cycle is complicated in most flat worms with one or more larval stages. In liver fluke miracidium, sporocyst, redia, cercaria and metacercaria larvae are present. In tapeworm onchosphere, hexacanth and cysticercus larvae are found.
14. Regeneration:
It is well marked in some flat worms like Planaria.
Unique Features:
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(i) Parenchyma,
(ii) Flame cells,
(iii) Ladder-like nervous system,
(iv) Self-fertilization occurs in some flat worms (e.g., Tapeworms).
Advancement over Ctenophores:
(i) Triploblastic.
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(ii) Organ-system level of organisation,
(iii) Gonoducts and copulatory organs.
Classification of Phylum Platyhelminthes:
Chiefly on the basis of mode of life, phylum platyhelminthes is divided into three classes.
Class 1. Tiirbellaria (L. turbella-stirring):
These are mostly free-living flatworms Examples. Dugesia (= Planaria).
Class 2. Trematoda (Gk. trema- hole + eidos = form):
These are ecto or endoparasitic worms (flukes) Examples. Fasciola, Schistostoma, Polystomum (endoparasite of the urinary bladder of frog and turtles).
Class 3. Cestoda. (Gk. kestos- girdle eidos = form):
These worms are endoparasites of vertebrates. The body is generally divided into a few to many proglottides (not true segments). Examples. Taeniar Echinococcus.
Dugesia (Planaria):
It is found in slow moving streams or fresh water ponds. It bears cilia on its body. It has a great power of regeneration. The heads bear two lateral lobes, the auricular processes, and a pair of eyes.
Schistosoma:
It is commonly called the “blood fluke” as it lives in the hepatic portal system and mesenteric blood vessels of human beings. Its intermediate host is snail. The male carries the female permanently in the gynaecophoric canal. Schistosoma shows well marked sexual dimorphism.
After fertilization the female leaves the male and lays eggs in the smaller blood Yessels. Schistosoma causes schistosomiasis. The symptoms of the disease are diarrhoea, anaemia, enlargement of liver and spleen, pain and fever. The disease can be prevented by not taking contaminated water.
Fasciola: The Liver fluke:
Adult Fasciola hepatica is found in the liver and bile duct of sheep, goat which are its primary hosts. The secondary or intermediate host is a pond snail (a mollusc) of genus Limnaea and genus Planorbis. The fluke is hermaphrodite but there is cross fertilization.
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It causes a disease known as liver rot (fascioliasis). The infected animal feels great muscular weakness resulting in muscular pain. The liver enlarges and the bile ducts are blocked. The disease may prove fatal to the animal.
Taenia solium— The Pork Tapeworm:
It is found in all those countries, where pork is taken as food. The adults of T. solium are parasites in the small intestine of human beings (primary host) and its larva is found in the muscles of the pig mostly (secondary host). The tapeworm does not have alimentary canal. The digested food of the host is diffused directly through the general body surface.
T. solium is hermaphrodite. There is self fertilization. The body is divisible into three parts: scolex, neck and strobila. At the top of the scolex lies a cone like rostellum. The rostellum bears at its base about 22-32 small curved chitinous hooks placed in two circlets. Hooks of the posterior circlet are somewhat smaller than those of the anterior one.
On the middle part of the scolex there are present 4 cup-like muscular suckers. The suckers and hooks are the organs of attachment. The strobila consists of immature, mature and gravid proglottides. Proglottides show pseudo metamerism (external segmentation only). Both mouth and anus are absent.
Tapeworm causes taeniasis. This disease can be characterized by such symptoms as abdominal pain, restlessness, anaemia, false appetite and indigestion.
Cysticercosis caused by cysticercus (bladder worm) is more dangerous than taeniasis. Cysticercus is a larva of tapeworm which develops from another larva, the onchosphere. Sometimes, the onchospheres reach human stomach directly with contaminated food and water or by antiperistalsis of intestine.
These onchospheres develop into bladder worms in man and thus, man becomes accidently intermediate host of the tapeworm. The bladder worms may reach the eyes where they may undergo encystment and cause blindness or in the brain they can cause epilepsy.
Taenia saginata (Beef tapeworm):
It lives in the human intestine of beef eating persons. Its intermediate hosts are cattle and buffaloes. It is longer than Taenia solium. Its body is divisible into scolex, neck and strobila. The scolex bears four adhesive suckers but it does not have rostellum and hooks. Its infection occurs by taking under cooked beef.
Echinococcus granulosus – The dog tapeworm or hydatid worm:
It lives as an endoparasite in the intestine of dogs, cats and foxes, etc. Man sometimes serves as an incidental host. The dog tapeworm consists of a scolex, neck and strobila. Storbila consists of 3 or 4 proglottides: one immature, one or two mature and one very large gravid.
Man acquires infection by playing with pet dogs. The hydatid worm causes hydatid disease in man. Presence of cysts of this worm in the brain and kidney may prove fatal.
