Sponges are probably the least animal-like of all animals. They are the most primitive of the multicellular animals. They lack all internal organs, including sensory organs and do not have any kind of neural system. They can almost better be described as a colony of single celled animals. However, they do have specialized cells that play specialized roles within the sponge.
Sponges are entirely immobile; all known species live affixed to some type of surface. There are a great many species and they grow in a variety of forms. Sponge shape can be classified into several rough groups: spherical, plate shaped, branching, conical, cylindrical, and plaque- or mat-like. What’s more, even within a single species, there can be great variation in the shape of individual colonies.
Almost all sponges are filter feeders, though ther are a few groups that feed by trapping tiny animals that come in contact with their sticky surface. Sponges have intricate networks of channels through their tissue. These channels are lined with small cells called choanocytes that synchronously wave their flagella to draw seawater through pores into the sponge. Small food particles are filtered from the water by other specialized cells before the seawater is ejected through larger openings.