Sponge, primitive animal of both marine and fresh water. Sponges are true animals, although they have only a simple body wall and no specialized organ or tissue system. They may be solitary or live in colonies. They are filter-feeders, straining tiny food particles out of water drawn in through pores all over the body surface and expelled through one or more vents. The body wall is strengthened by spicules of calcite or silica, or by a meshwork of protein fibers called spongin. Sponges with spongin skeletons are fished for use as bath sponges.