Question

Which of the following phylum do not possess true coelom?
(a) Aschelminthes
(b) Annelida
(c) Ctenophora
(d) Platyhelminthes

Answer 1

 
Hint: A coelom is the cavity present in the body of many animals and is positioned such that it surrounds and contains the digestive tract and the other internal organs. It can be a true cavity or a false cavity. A false cavity or pseudocoelom is found in the roundworms.

Complete Step by Step Answer:
An organism can be a true coelomate, an acoelomate, or a pseudocoelomate depending on the splitting of the embryonic mesoderm. The true coelomate organisms are said to have a tube within a tube body plan.
A pseudocoelomate or a blastocoelomate is the invertebrate organism with a three- layered body structure with a pseudocoel. The only phylum of animals to possess a false coelom or a pseudocoelom is the Aschelminthes or the roundworms which include organisms such as Ascaris.
In these protostomes, the embryonic blastocoel persists as a body cavity. These animals have a fluid- filled main body cavity that is either unlined or partially lined with tissue derived from mesoderm. This space serves many functions including the distribution of nutrients and the removal of waste materials, and supporting the body as a hydrostatic skeleton.
So, the correct answer is, “Aschelminthes”.

Note:
- Aschelminthes is the only phylum with a pseudocoelom.
- The phylum Annelida has a true coelom with a completely split mesoderm instead of patches of mesoderm as seen in the pseudocoelomates.
- The phylum Platyhelminthes is acoelomate, i.e. the mesoderm is not at all split in these organisms and they possess a three- layered body structure.
- The phylum Ctenophora has two layers of cells with a poorly defined third layer called mesoglea in between them. A true coelom is absent in them too.