Top left. Acoelomates lack enclosed body cavities. Top right. Pseudocoelomates have a body cavity lined on the outside with muscle cells deriving from the embryonic mesoderm. Left. In coelomates, both the outer and inner sides of the cavity are lined by cells deriving from mesoderm. For a quick review of the formation of embryonic tissue layers, see Figure 32.3 (page 622) of your text.