Pseudocoelomates

Pseudocoelomates are animals that possess a body cavity called a pseudocoel, which lies between the gut and the outer body wall and is not entirely lined by mesoderm. They are traditionally described as a group that lacks a true coelom, in contrast to coelomates whose coelom is fully lined by mesoderm and aceolomates that lack a body cavity. The pseudocoel is thought to originate from the blastocoel during development, and its lining is incomplete, with mesoderm surrounding the outer body wall and internal organs while the gut remains lined by endoderm.

The pseudocoel serves as a hydrostatic skeleton that supports movement and helps distribute nutrients within the body. Many pseudocoelomates lack a dedicated circulatory system; transport of materials occurs primarily by diffusion within the fluid of the pseudocoel. The gut is typically complete, but contact between gut tissues and the body wall is limited by the pseudocoel, and gas exchange is mainly through diffusion across the body surface or cuticle.

Representative members most commonly cited as pseudocoelomates include nematodes (roundworms) and rotifers, which are the best understood. Other groups historically listed in this category include gastrotrichs, acanthocephalans, and related phyla. A notable feature in some pseudocoelomates, particularly nematodes, is eutely, the constant cell number that characterizes development in adulthood.

Evolution and classification notes: pseudocoelomates are not a monophyletic clade but a traditional, informal grade. With advances in molecular phylogenetics, many such taxa are reinterpreted within broader protostome lineages, and the term emphasizes a structural rather than strict evolutionary grouping.