A coelom (Greek: coel = hollow cavity) is a fluid-filled cavity between the alimentary canal and the body wall lined on all sides by mesoderm. The peritoneal cavity in our abdomen is one part of our coelom and there are similar spaces around our heart and lungs. However, the type of coelom (or even its existence) differs among groups of animals both in its structure and mode of development. There are three structural types of body plans related to the coelom.

1. Acoelomates, in which no coelomic cavity exists. Examples are flatworms (Platyhelminthes), coelenterates and sponges. Only a gut, coelenteron or spongocoel exists in these animals and there is no other cavity.

2. Pseudocoelomates, in which a body cavity exists in addition to alimentary canal, but it is lined by mesoderm only on the outer body wall side and not around the gut. Examples are round worms (Nemathelminthes) and some minor phyla grouped under Aschelminthes.

3. Coelomates or Eucoelmates.  They are true coelomates in which the coelom is lined both on the inside of the body wall as well as around the gut by mesoderm. Animals with a true coelom also have mesenteries, which suspend the body organs within the coelom. Animals higher to round worms such as annelids, arthropods, mollusks, echinoderms and chordates fall in this category. Tue coelomates are of the following types.

(i) Schizocoelomates are true coelomates in which the body cavity originates by splitting of mesodermal tissue at the time of gastrulation. This method of coelom formation is called schizocoelous (Greek: schizo = split), and occurs in animals like annelids, arthropods and mollusks. Sometimes the schizocoelom is filled with blood and is called haemocoel as in arthropods and mollusks.

(ii) Enterocoelomates. In most deuterostomes, such as chordates and echinoderms, the coelom originates by out-pouching of the archenteron during gastrulation. Each pouch then expands and its mesoderm lines the gut on the inner side and body wall on the outer side. This method of coelom formation is called enterocoelous.

Segmentation, Cephalization and Tagmosis

Segmentation, also known as metamerization, is the structural grouping of parts of an animal body into discrete segments. Cephalization means that there is a head, and therefore a concentration of sensory organs, feeding organs, and centers of neural integration near the anterior end of the animal. While at first seeming a bit simplistic, cephalization has tremendous implications for animals. Tagmosis occurs in segmented animals where groups of segments are organized into functional units. A good example is in arthropods, where segments are grouped into body regions like the head, thorax, and abdomen, each having its own suite of functions.