Q. 3. What is coelum? Is it found in Platyhelminthes? What has happened to it in Platy helminthes?
Coelum is perivisceral cavity which encloses visceral organs. It is formed by the splitting up of mesoderm and is, therefore, a space between somatic and visceral layers of mesoderm. It is lined by parietal epithelium or parietal peritoneum on the outside and by visceral epithelium or visceral peritoneum on the inner side. It is filled with coelomic fluid. The reproductive organs and excretory organs develop from its wall.
Platyhelminthes neither possess a body cavity nor a coelom. Though, these are triploblastic, in them the mesoderm is not divided into somatic and splanchnic layers. Rather it forms a loose parenchyma or mesenchyma. Therefore, in flatworms the space between body wall and alimentary canal is filled with the loose parenchyma and coelom is not formed.
BSc 1st Year Lower Non-chordates Helminths Sample Model Practice Question Answer Papers