pseudocoeloom

Pseudocoelom, sometimes spelled pseudocoeloom, is a fluid-filled body cavity located between the mesoderm-derived body wall and the endoderm-derived gut in certain animals. It is not a true coelom because its lining is incomplete; in pseudocoelomates, mesoderm lines the outer body wall and parts of internal organs, but the gut remains lined by endoderm rather than mesoderm.

Developmentally, the pseudocoelom is often described as a persistent blastocoel or as a cavity formed by limited mesodermal contribution during early development. The cavity is typically not completely lined by mesoderm, distinguishing it from a true coelom formed by schizocoely or enterocoely.

Structure and function: The pseudocoel contains a fluid that provides hydrostatic support, allowing a degree of locomotion in many pseudocoelomates, most notably nematodes (roundworms). It also serves as a storage and transport space for nutrients and waste, and houses some internal organs. The gut, derived from endoderm, lies within the cavity and is not mesodermally lined.

Examples: Nematodes (roundworms) are the most prominent pseudocoelomates. Other groups historically associated with pseudocoelomates include rotifers and some microscopic worm groups such as gastrotrichs, though modern classifications vary and some of these groups are reorganized in contemporary anatomy.

Relation to other body cavities: A pseudocoelom differs from a true coelom, which is completely lined by mesoderm. Coelom formation occurs by schizocoely or enterocoely during embryonic development. The term pseudocoelom is therefore used to describe a non-true, body cavity with limited mesodermal lining.