Sponges are primitive, sedentary animals which lack a mouth, digestive canal or any other normal organ. They consist of a few types of cells, forming tissues that are supported by a skeleton made of a fibrous material namely collagen or spongin(as in the familiar bath sponge), or of lime or silica spicules, or of a combination of spongin and spicules. The type of skeletal structure and and form of spicules play an important role in the classification of sponges. Water and small food particles enter the sponge through numerous tiny pores which dot the surface of the body and exits through one or a few larger openings or oscula (often raised like turrets). Collar cells line the body cavity and beat their hair-like flagellae to generate the water flow and filter out food particles through the net-like collars. Sponges reproduce either by budding or by producing planktonic lavae.
Charateristics:
1. Symmetry radial or none. Multicellular. Cells imperfectly arranged as tissues with mesenchyme in between.
2. Body with many pores, canals, or chambers through which water flows.
3. Some or all interior surfaces lined with choanocytes (flagellated collar cells).
4. No organs, movable parts, or appendages. Digestion intracellular.
5. Usually with an internal skeleton of seperate crystalline spicules or of irregular organic fibers, or both.
6. Reproduction asexual by buds or gemmules, also sexual by eggs and sperm. Larva ciliated, free swimming.
7. Development of larva unique, with external flagellated cells moving inside and internal cells outward.
A diagram of a partially sectioned, generalized sponge
