The 5000 species of sponges are a primitive group of aquatic filter feeders found on suitable hard surfaces such as rocks and submerged wreckage. At one time they were incorrectly considered to be plants because of their lack of discernible movement. Mostly marine organisms, a few are successful fresh water colonists.
Sponges have evolved little since their first appearance in the fossil record some 800 million years ago. They appear to be an evolutionary blind alley and outside the main streams of animal evolution. In spite of this, their simple body plan has allowed the group to develop a wide range of body shapes and sizes.