These animals lack true tissue, which is a characteristic that all other animal phyla have. Sponges are sessile animals that live in fresh or salt water. There are approximately 5000 salt water species and 150 fresh water species. Most sponges, except for a few deep sea species, live in shallow water attached to rocks, submerged timbers, shells, coral, or in sand or mud bottoms. Sponge structure is unique in that it is constructed around a series of canals.
Sponges lack any kind of specialized organs but instead have well-developed connective tissue.
Specialized cells within the connective tissue carry out different functions. Sponges are identified by their spicules or skeletal structure, located in the mesohyl layer. Spicules can be calcareous, siliceous or made of spongin fibers. External structures Sponges vary greatly in size and can be found as small as a grain of rice to exceeding a meter in height and diameter. Most sponges are irregularly shaped and have erect, massive branching growth patterns.