Acoelomate versus Coelomate body structures

A coelom is a hollow space surrounded by tissues. The basic structure of most animals comprises three tissue layers, an interior endoderm and an exterior ectoderm that are separated by a mesoderm. Typically, the endoderm forms the innermost digestive tract, the ectoderm the outermost skin layer, and the remaining internal organs are formed from the mesoderm. In derived animals, the endodermal tube is suspended from the dorsal and ventral surfaces by mesoderm, so as to leave open coeloms that allow circulation of fluids. Advanced circulatory systems develop from these coeloms.

Figure ©1999 Campbell et al.;  text © 2019 by Steven M. Carr