When body cavity is not completely lined by the mesoderm instead it is present in the form of scattered pouches, in between ectoderm and endoderm, this type of body cavity called pseudocoelomate, e.g., roundworm.

The animals in which coelom is completely absent are called acoelomates, e.g., flatworms

Animals with body cavity lined by mesoderm and hence have true coelom, are called coelomates. E.g., annelids

The animals in which body cavity is filled with haemolymph, are called haemocoelo mates e.g., arthropods