Body Plans of Animals

Cnidarians are composed of only two cellular layers, an outer epidermis and an inner gastrodermis. In cnidarians these cell layers are thin and the animals are therefore very fragile. Animals above the cnidarians on the evolutionary tree (Appendix A, Figure A.1) have three layers of cells. Each layer gives rise to specific structures within the organism. The ectodermis is responsible for the production of much of the sensory system and surface skin (among other things), while the endoderm contributes to the growth of structures such as the lining of the gut. The third cell layer lies between the ectoderm and endoderm and is called the mesoderm). This third layer is used to build muscular, excretory, and reproductive systems. The mesoderm thus allows animals to become more organized in their structure. Cnidarians, for example, are organized mainly at the level of tissues, while those with three layers possess organs and elaborate organ systems.

There are three ways bodies can be organized around the cellular layers. The simplest organization (seen in the flatworms) is the acoelomate body plan. In these animals the body cavity is loosely filled with mesodermal cells called parenchyme. The pseudocoelomate body plan has a fluid-filled cavity called the pseudocoelom. Members of the aschelminth phyla are pseudocoelomate in structure. The final organizational level (seen in annelids and all other phyla above the aschelminths) is the coelomate body plan. Coelomates have the body cavity (the coelom) lined with mesodermal tissue (the peritoneum). The peritoneum surrounds all organs in the body cavity. In some coelomates the coelom may be present only in larval stages or may be greatly reduced.