PLATYHELMINTHS

n 3.1 MAIN CHARACTERISTICS

n Three body layers: ectoderm, mesoderm and endoderm without a body cavity;

aceolomate (without a body cavity)

Main body structure:

n Acoelomate; dorso-ventrally flattened; bilaterally symmetrical; no anus.

n Mesoderm develops into a specialised connective layer known as the parenchyma.

n The organs — reproductive, nerves, muscular, excretory etc, are embedded within

the parenchyma.

n No specialised respiratory organs. Gaseous exchange is by diffusion through outer

body layer.

n Specialised excretory ‘organs’ known as flame cells unique to this phylum. These cells

link into a tubular system for the removal of liquid nitrogenous waste.

n Nearly all members of this phylum are hermaphrodite.

n 3.2 TURBELLARIANS

Most of the turbellarians are free-living (about 3,000 species) and are found in

marine, freshwater and moist terrestrial habitats. About 150 species are commensal or

parasitic.

n They have ciliated outer layer.

n The average length is about 1 cm.

n Simple digestive system with variations within the group.

n They have a simple mouth opening with no well formed gut cavity. The food is

packed into a dense mass of digestive cells.

n In the majority the mouth opens into a gut which may be straight, three-branched or

multi-branched. More advanced species have a protrusible pharynx; some have a pro-

trusible pharynx that can ‘spear’ food items.

n 3.3 CESTODES

A unique feature of this group is that the adults only occupy one type of habitat — the

gut or derivatives such as the bile duct or pancreatic duct of a vertebrate.

3