Skeleton of Sponge
In zoology a skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. The mesohyl functions as an endoskeleton in most sponges, and is the only skeleton in soft sponges that encrust hard surfaces such as rocks. More commonly the mesohyl is stiffened by mineral spicules, by spongin fibers or both. Portion of a wall of sponge (fig 17) showing the outer layer (e), the inner layer (i), consisting of collared cells, and the middle layer (m), consisting of irregular cells.