pseudocoel

A pseudocoel, or pseudocoelom, is a fluid-filled body cavity found in several invertebrate phyla. It lies between the endoderm-derived gut and the mesoderm-derived body wall. Unlike a true coelom, the pseudocoel is not completely lined by mesodermal peritoneum on its inner surface; the outer body wall and many internal organs are mesodermally derived, while the inner boundary around the gut is formed from other tissues. Embryologically, the cavity commonly derives from the blastocoel, so the resulting space is the so-called false coelom.

Pseudocoelomates include nematodes (roundworms), rotifers, gastrotrichs, and some other invertebrate phyla such as acanthocephalans. In these animals, the pseudocoel houses a fluid that serves as a hydrostatic skeleton, supports the distribution of nutrients throughout the body, and provides space for organs such as the reproductive and excretory systems. The gut occupies part of the cavity, and many tissues are suspended within it by connective tissue strands. The pseudocoel can also play a role in locomotion and the circulation of materials within the organism.

In modern taxonomy, pseudocoelomates are a historical grouping rather than a single clade; the presence of a pseudocoel has evolved independently in several lineages and does not indicate a true coelom. The pseudocoel thus represents a characteristic that contrasts with the true coelom found in many other animals, where a mesoderm-derived lining fully encloses the body cavity and its organs.