What characterizes pseudocoelomates?

Pseudocoelomates are organisms that possess a pseudocoel, which is a body cavity that is not entirely lined with mesoderm. Instead, only the outer wall of the cavity is lined by mesoderm, while the inner wall is lined by endoderm. This distinguishes them from coelomates, which have a true coelom lined completely by mesoderm tissue. The presence of a pseudocoel provides certain functional advantages, such as allowing for the development of various organ systems and facilitating movement by enabling a more complex body structure that can support various functions, including circulation and hydrostatic pressure for movement.

This characteristic differentiates pseudocoelomates from organisms that lack a body cavity completely, such as acoelomates, and from those with a true coelom, which is a more developed body cavity fully lined with mesoderm. The option referring to no body cavity and the option regarding a true body cavity do not accurately describe pseudocoelomates, while the mention of an external protective shell does not relate to the classification based on body cavity type.