Figure 3: Body cavities of animals become increasingly more complex as one ascends the evolutionary tree. Ctenophora and Cnidaria are diploblastic (a) and have two thin but well-differentiated tissue layers separated by mesoglea, a gelatinous material that protects the body and lines the gut. Flatworms have three primary tissue layers and are triploblastic but are without an inner cavity called a coelom (b). Ectoderm is the outer tissue layer, mesenchyme and mesoderm are the middle tissue layers, and endoderm is the inner layer; mesoderm is the tissue layer that produces most muscles. Flatworms lack a circulatory system, so oxygen must be transported to their inner tissue layer by diffusion, and thus their bodies must remain flat. Animals with the next most complicated body plan have three tissue layers as well as hemocoelic, or blood, spaces between the tissue layers (c). The most complicated body plans have both hemocoelic spaces and coelomic spaces, which develop inside the third or innermost layer (d). In general, animals with only blood spaces are found in lower branches of the tree than are animals with coelomic spaces. (Illustration by Linda Huff.)

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