True enterocoelic coelom appeared first in the course of evolution?
Hint: True coelom is basically defined as a secondary body cavity that is formed by the separation or splitting of mesoderm during the embryonic development. Coelomates are the one which have coelom with a complete lining that is called as peritoneum that is derived from the layer mesoderm.
Complete answer: Coelom is basically a body cavity that is present in between the alimentary canal and the body wall which is lined by the layer mesoderm and it is filled with the fluid. Coelom allows the compartmentalization of body parts. We can also refer to it as a “secondary body cavity”. They also play a very important role in the protection of organs from shock as well as compression. Most of the bilateral animals are coelomates in nature. Enterocoelomates is an animal in which the body cavity is lined by the mesoderm, that means the coelom arises in the embryonic stage as an outpocketing of the developing gut or we call it enteron. This kind of development can be seen in the echinodermata and also found in few other phyla and in chordates as well. It first appeared in Echinoderms during the course of evolution.
Therefore, the correct answer is “Echinodermata”.
Note: There are basically three types of animals based on coelom and that are acoelomates, pseudocoelomates and coelomates. Acoelomates are those without coelom, for example - Platyhelminthes. Pseudocoelomates are those whose body cavity is not lined by mesoderm, for example - Aschelminthes. And coelomates are those which have a true body cavity, for example – Annelid, Arthropods etc.