How is a pseudocoelomate defined?

A pseudocoelomate is defined as an organism that possesses a body cavity called a pseudocoel, which is a fluid-filled space located between the digestive tract and the outer body wall. This cavity is not entirely lined by mesodermal tissue; instead, it is only partially lined. The presence of this cavity allows for greater internal mobility and the development of more complex organ systems compared to organisms without such cavities.

The characteristic of being a pseudocoelomate specifically highlights that while there is a body cavity, it does not have the complete lining of mesoderm that is found in true coelomates. Thus, the body cavity in pseudocoelomates is somewhat less complex but is important for physiological processes. This allows for the accommodation of organ systems without the rigidity of a fully muscular lining, providing advantages in terms of movement and flexibility.