Acoelomate

Acoelomate
("A-see-lem-ate") An invertebrate lacking a coelom (fluid-filled body cavity) where usually the digestive cavity is the only internal cavity.

Acoelomate organs are not protected from crushing forces applied to the animal’s outer surface, hence why the majority of acoelomate organisms live some type of liquid environment and utilize diffusion.

Examples:

 * Porifera
 * Cnidaria
 * Platyhelminthes

See also: Development & Tissues