Acoelomates are animals that lack a body cavity or coelom between their digestive tract and outer body wall. They have a solid body with tissues filled in the space where a coelom would be.
Coelomates: Animals with a true body cavity completely lined by mesodermal tissue.
Pseudocoelomates: Animals with a body cavity that is not entirely lined by mesodermal tissue.
Bilateral Symmetry: A type of symmetry where an organism can be divided into mirrored left and right halves along one plane.