Question

True coelom has evolved for the first time in
(a) Arthropoda
(b) Mollusca
(c) Annelida
(d) Platyhelminthes

Answer 1

Hint: True coelom is defined as a secondary body cavity formed by the splitting of mesoderm during embryonic development. Coelomates have coelom with a complete lining called peritoneum derived from mesoderm.

Complete answer:
- Phylum Annelida shows evidence for first true coelomates
- Coelom is the body cavity present in between the alimentary canal and body wall which is lined by mesoderm and is filled with fluid.
- In true coelom, the coelom is derived from Archenteron.
- It is found in Phylum Annelida, Phylum Arthropoda, Phylum Mollusca, Phylum Echinodermata, Phylum Hemichordata, Phylum Chordata.
- True coelom is of 2 types- Schizocoelom and Enterocoelom.

Additional information:
- Coelom allows the compartmentalization of body parts.
- They play a major role in protecting organs from shock as well as compression.
- Sometimes true coelom is referred to as ”Secondary body cavity”.There are excretory organs that open into it as well as many reproductive organs arise from its wall.
- Coelom is not present in Phylum Porifera, Phylum Cnidaria, Phylum Ctenophora, Phylum Platyhelminthes.
- Pseudocoelom is present in Phylum Aschelminthes. In this, a false coelom is present in the form of scattered pouches which are not lined by mesoderm.
- Most bilateral animals are coelomates.
So the correct answer is ‘(c) Annelida’.

Note: Having a coelom is an advantage to the circulatory system because it makes it easier for nutrients to move throughout the body. Phylum Annelida is the first phylum to have a closed circulatory system. The term coelom was suggested by Haeckel in 1872. In coelomates there is a colorless coelomic fluid and the excretory organs open into it.