Phylum Platyhelminthes - The flat worms

The flat worms are mostly parasites but some are free living e.g., Planaria

Acoelomate, triploblastic, bilaterally symmetrical and dorsoventrally flattened animals with organ system level of organisation.

Body segmented (false segmentation) except in Class Cestoda.

Body covered with a cellular, syncytial one-layered partly ciliated epidermis; while in parasitic trematodes & cestodes epidermis is lacking & the body is covered with cuticle.

Exoskeleton & endoskeleton are completely absent. However hooks, spines, suckers, teeths may be present which act as adhesive organs.

The space between the body wall, alimentary canal & other organs is filled with a peculiar connective tissue, called the parenchyma. It helps in transportation of food materials.

Digestive system is totally absent in tapeworms but in other flatworms (Trematoda & Turbellaria) it consists of mouth, pharynx & blind intestine (anus absent).

Respiratory & circulatory systems are absent.

Excretory system consists of single or paired protonephridia with flame cells.

Nervous system is primitive. The main nervous system consists of a pair of cerebral ganglia or brain and one to three pairs of longitudinal nerve cords connected to each other by transverse commisures. This type of nervous system is called ladder type of nervous system.

Sense organs are of common occurrence in Turbellaria but these are greatly reduced in parasitic forms.

Sexes are united, i.e., hermaphrodite with very few exceptions.

Asexual reproduction by fission occurs in many fresh water Turbellarians

In majority of forms, eggs are devoid of yolk but provided with special yolk cells and are covered by egg shell.

Cross fertilization in trematodes and self-fertilization in cestodes is very common. Fertilization is internal.Development incomplete. Life-cycle complicated, involves one or more hosts.

Classification :

The Phylum Platyhelminthes is divided into three classes : Turbellaria, Trematoda and Cestoda.

1. Turbellaria

Characters :

Body is leaf-like and covered by a delicate, partly or fully ciliated epidermis.

Alimentary canal is usually present and mouth is often ventral.

Life-history is simple.

The turbellarians are mostly free-living.

Examples : Dugesia or Planaria.

2. Trematoda

Characters

Body is leaf-like and covered by a thick, living tegument. There is no epidermis.

Alimentary canal is present and mouth is anterior.

Life-history often includes larval forms.

The trematodes are parasitic flatworms. They are commonly called the flukes.

Usually hermaphrodite or unisexual.

Examples : Fasciola-the liver-fluke, Schistosoma-the blood fluke.

3. Cestoda

Characters

Body is ribbon-like and covered by a thick, living tegument. There is no epidermis.

Alimentary canal and mouth are absent.

Life-history includes larval stages.

The cestodes are intestinal parasites. They are commonly called tapeworms.

They have suckers and hooks for attachment in the host.

Every segment is hermaphrodite.

Example :

Taenia saginata (beef tapeworm), Taenia solium (Pork tapeworm), Echinococcus granulosus (Smallest tapeworm).

Further Reading : 

1. Animal Kingdom

2. Exercise for Animals non Chordates