Sponges are the simplest of many-celled animals; the dividing line between plant and animals. They have a wide variety of sizes, shapes and colors, making them a very diversified group.

A sponge consists of a cooperating community of individual cells, each performing specific functions. The cells surround a system of canals which pump water through the sponge. Water enters through ostia and are pushed through chambers by beating flagellum where food and oxygen are siphoned out and the water exits through the osculum. Simple sponges only have one chamber and osculum, while more complex sponges have many chambers and osculums. Most sponges have a skeleton made of spongin, spicules, or a combination of both. They reproduce both sexually and asexually through budding and gemmules. They also usually have both male and female reproductive organs.

There are three classes in this phylum: Calcispongiae, Hyalospongiae, and Demospongiae.