Most animals are motile during at least part of their life cycle. Cephalization means having a definite head end, usually with feeding and sensory features. The segments may be grouped and modified for specialized tasks, as for example in insects. Sponges have an asymmetric body with no true tissues, no organs. Flattened cells cover the exterior.
Collar cells line the interior chambers.
By means of their beating flagella, these cells move large volumes of water in through body pores and out through the large osculum at the top of the body. They also trap suspended food particles in their microvilli collars and transfer the food to amoebalike cells in the matrix. Between the two layers of cells there is a semifluid matrix with needlelike spicules for support. Sponges reproduce sexually and asexually. Sperm are released into the surrounding water to be picked up by a nearby sponge and directed to the egg within the matrix.