Ans: The coelom is the fluid-filled or body cavity space followed by the mesoderm. Animals possessing coelom are called coelomates. The body cavity in some animals is not filed by mesoderm; instead, it is presented as deranged pouches in between the endoderm and ectoderm. This body cavity is called pseudocoelom, and the animals who are possessing them are known as pseudocoelomates, for example – aschelminthes. Animals whose body cavity is absent are referred to as acoelomates, example – Platyhelminthes. The classification of coelom and body cavity are essential to determine the complexity of an animal at the organ level.