Pseudocoelom

Pseudocoelom is a fluid-filled body cavity characteristic of pseudocoelomates. It lies between the endodermal gut and the mesoderm-derived body wall, and it is not completely lined by mesodermal tissue as a true coelom is.

During embryonic development, the pseudocoelom typically forms from the blastocoel and persists as the main internal cavity, whereas true coeloms form later in other animals by schizocoely or enterocoely. Consequently, the gut is not surrounded by a complete mesodermal lining.

Functionally, the pseudocoelom acts as a hydrostatic skeleton, enabling movement through contraction of the body wall muscles. It provides space for several organs, including parts of the reproductive and excretory systems, and enables diffusion and sometimes circulation of nutrients and wastes in the absence of a true circulatory system.

Taxonomically, pseudocoelomates include nematodes, rotifers, and gastrotrichs. The concept contrasts with true coelomates, which possess a fully lined coelom, and acoelomates, which lack a body cavity. In modern classifications, the category pseudocoelomate is largely historical, yet the pseudocoelom remains a useful descriptive feature in animal anatomy and development.