Body cavity not completely lined by the mesoderm, instead present in the form of scattered pouches, in between ectoderm and endoderm, is called pseudocoelomate, e.g., roundworm.
The animals in which coelom is completely absent, e.g., flatworms are acoelomates.
Coelomates have their body cavity lined by mesoderm and hence have true coelom, e.g., annelids, molluscs, arthropods, etc.
Haemocoelomates are the animals in which body cavity is filled with haemolymph, e.g., arthropods, molluscs.