(a) Bilateral symmetry: When the body is divided into identical left and right halves in only one plane, it is called bilateral symmetry. E.g., Annelida

(b) Coelom: It is the body cavity which is filled with a fluid and is lined by mesoderm. Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms.

(c) Triploblastic: Animals having three germinal layers (ectoderm, mesoderm and endoderm) are called triploblastic. Example: Nematoda.