pseudocoelomate

A pseudocoelomate is an animal that possesses a pseudocoel, a fluid-filled body cavity located between the endodermal gut and the mesoderm-derived body wall. The inner surface of this cavity is not lined by mesoderm, as in true coelomates; instead, the gut remains endodermally lined, and the surrounding mesoderm forms the outer body wall and suspends internal organs. Most pseudocoelomates have a complete digestive tract and rely on the fluid-filled cavity as a hydrostatic skeleton to support movement and shape.

In pseudocoelomates, the pseudocoel serves as a circulatory-free space in which tissues and organs develop and function. The nervous system and excretory structures are typically simplified, and gas exchange usually occurs by diffusion. Because the body cavity is not fully lined by mesoderm, these animals have a simpler body plan than true coelomates.

Taxonomic status: The term pseudocoelomate refers to a grade of organization rather than a monophyletic group. The best-known examples are nematodes (roundworms) and rotifers, with other groups such as gastrotrichs and acanthocephalans often cited. Modern systematics place these lineages within separate clades, acknowledging that the pseudocoelomate condition evolved independently in different lineages.

Evolution and significance: The pseudocoel allows a flexible, expandable body plan and a simple hydrostatic skeleton, but it offers less architectural specialization than a true coelom. The pseudocoelomate condition illustrates alternative strategies for digestion, circulation (diffusion-based), and movement in small invertebrates.