pseudocoelomate , any of a group of invertebrates with a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers). The pseudocoelom is contrasted with the coelom of mollusks, annelid worms, and the more complex animals (including humans and other vertebrates). Pseudocoelomates lack a circulatory system, and the pseudocoelom itself lacks the endothelial lining of a coelom. The hydrostatic pressure of the pseudocoelom gives the body a supportive framework that acts as a skeleton. Nematodes or roundworms (see Nematoda ), rotifers (see Rotifera ), acanthocephalans (spiny-headed worms), kinorhynchs (see Kinorhyncha ) and nematomorphs or horsehair worms (see Nematomorpha ) are pseudocoelomates.