The body cavity of the organisms is lined by a layer of cells called mesoderm. This body cavity is called a coelom. This term coelom is coined by the scientist “Haekel”. To be more elaborate, The space between the body wall of organisms and the visceral organs is filled with fluid. This space is called Coelom.Complete Answer:
The organisms are classified into various types based on the different types of coelom.
1. Acoelomates: They do not contain any body cavity hence the organisms having no coelom are called Acoelomates. Example : Platyhelminthes
2. Eucoelomates: They contain a true body cavity filled with fluid lined by mesoderm epithelial cells. Hence the organisms containing true coelom are defined as acoelomates.
Examples: annelida, Mollusca etc.
They contain the body cavity filled with fluid but they are not lined by the mesodermal epithelial cells. Hence, it is considered a false coelom and the organisms are called pseudocoelomates.
Examples: Aschleminthes - Nematoda, rotifera.Hence, The phylum under which the pseudocoelomates are placed is Nematoda/Aschelminthes.
Phylum nematoda consists of two classes, namely Aphasmidia, example: Trichinella and Phasmidia, examples Ascaris.Note:
Pseudocoelom is considered as the false body cavity. Tube within tube organization in organisms started with pseudocoelomates only. It functions as normal coelom only but some muscular movements of organs present internally may be restricted. It also acts as a shock absorber which means it protects organs inside from any external force.