A pseudocoelom is a type of body cavity found in certain animals, particularly in the phylum Nematoda (roundworms) and some Rotifera. Unlike a true coelom, which is lined with mesoderm, a pseudocoelom is not completely surrounded by mesodermal tissue. Instead, it is a fluid-filled space between the endoderm and mesoderm, providing structural support and allowing for the movement of organs. This body cavity plays a crucial role in the physiology of pseudocoelomates, as it helps in the distribution of nutrients and waste products. The presence of a pseudocoelom allows these organisms to maintain their shape and facilitates movement, making it an important evolutionary adaptation for many invertebrates.