Introduction to Platyhelminthes:

The Platyhelminthes are the simplest animals which are bilaterally symmetrical and triploblastic (that is, comprised of three basic cell layers). They are flatworm's deficient body cavity other than the gut and lack an anus; the similar pharyngeal opening both takes in food and ejects waste. Because of the deficiency of any other body cavity, in bigger flatworms the gut is often very highly branched in order to move food to all portions of the body. 

The deficiency of a cavity as well constrains flatworms to be flat; they should respire through diffusion, and no cell can be too far from the outside, preparing an essential flattened shape.

Platyhelminthes have practically no fossil record. Some trace fossils have been reported which were almost certainly made up by Platyhelminthes, and fossil Trematodes eggs have been found in Egyptian mummies and in the dried dung of the Pleistocene ground sloth. Trematodes larvae which parasitize mollusks might leave pits or thin spots within the shell and such pits might be acknowledged on fossil shells. When the mollusk is irritated by the presence of Trematodes larvae, it might be capable to surround them with layers of shelly material - and therefore do parasites become natural pearls. 

Features of Platyhelminthes:

1) Flat worms are triploblastic that is, having three germ layers. They are the primary organ grade of organization.

2) Body is dorsoventrally flattened and consists of a definite anterior end, distinguished as a head.

3) They are acoelomate animals. The space among the body wall and gut are filled by parenchyma comprised of loose cell masses.

4) Body is without true metameric segmentation.

5) Alimentary canal is generally highly branched; it opens out through a single opening of the mouth. Anus is not present.

6) Circulatory and respiratory system is not present.

7) Nervous system is similar to ladder. It is build up of brain and two main longitudinal nerve cords joined at intervals by transverse commissures.

8) Excretion is functioned by specialized cells termed as flame cells. They lead to the tubules that open out by one or more excretory pores.

9) Reproductive system is well-ordered. Most of them are hermaphrodites.

Classes of Platyhelminthes: Turbellaria, Trematoda and Cestoda

Turbellaria:

The class Turbellaria comprises all free-living members of the phylum and also a few parasites. It comprises numerous marine forms, whose beautiful colors serve as the warning of their toxicity to would-be predators and also the more drab freshwater planarians (Dugesia). A few Turbellaria can swim through undulations of the body margins; however most of them glide elegantly over surfaces all along a trail of mucus, pushed through cilia on the ventral surface.

Trematoda:

The class Trematoda, generally termed as flukes, are unsegmented parasitic flatworms which generally parasitize a snail as an intermediate host (that is, in which they reproduce asexually) and a human or other vertebrate as a definitive host (that is, in which the worms mate and lay eggs). Most of the species have other hosts among these two, like frogs or fish. Trematodes generally encompass a pair of suckers for crawling and clinging to the host's tissues. Most of the humans are infected with blood flukes, lung flukes, liver flukes and other Trematodes parasites of great medical significance.

Cestoda:

The Cestoda, generally termed as tapeworms, are segmented, ribbon like parasites generally found as adults in the small intestines of vertebrate animals. Dissimilar the other classes, they have no digestive region, for they can absorb predigested nutrients from the host's intestine. The body comprises of a long chain of segments, each having its own reproductive system. The anterior end is a knoblike holdfast termed as a scolex, equipped with suckers and often hooks for attachment to the host's intestine. In common, tapeworm infections are not as medically serious as Trematodes infections; however some tapeworms can be toxic.

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