A quasicoelomate is an organism that possesses a body cavity that is not a true coelom. Instead of being completely lined by mesodermal tissue, the body cavity is partially lined or is a pseudocoelom, which may be derived from the blastocoel. This term is often used to describe certain invertebrates, such as those in the phylum Rotifera, where the body cavity allows for the development of organ systems while lacking the full structural support of a true coelom.