The Probert Encyclopaedia of Nature

SPONGE

Sponges (Porifera) are a group of simple animals. They possess a porous ' spongy' texture and no definite external organs or form. They vary in shape and hardness. The sponge body consists of a mass of cells of various kinds forming a delicate tissue, and this is supported on a skeleton of minute rods, or spicules, of calcite, silica or of a horny organic substance.
Research Sponge