BODY PLANS IN HIGHER ANIMALS

            There are three ways bodies can be organized around the cellular layers.  The simplest organization (seen in the flatworms - Platyhelminthes) is the acoelomate body plan.  In these animals the body cavity is loosely filled with mesodermal cells called parenchyma.  The pseudocoelomate body plan has a fluid filled cavity called the pseudocoelom.  The internal organs of pseudocoelomate animals are actually found free within the fluid-filled body cavity (pseudocoelom), as there is no layer of mesodermal epithelium lining the cavity and surrounding the organs.  The final organizational level (seen in the annelids and all other phyla above) is the coelomate body plan.  Coelomates have the body cavity (the coelom) lined with mesodermal tissue which surrounds all organs in the body cavity (Figure 14).

Figure 14: Body plans.