Phylum Porifera

The phylum Porifera (Latin porus, pore + ferre, to bear) comprises the sponges, the simplest and most primitive of the multicellular animals, standing at the cellular level of organisation — a sponge's component cells carry out feeding, digestion and other functions with a large degree of individual autonomy, without being welded into true tissues. Sponges are overwhelmingly marine, although a few families such as Spongillidae live in fresh water, and the adult body is sessile, permanently fixed to a hard substratum, and generally asymmetrical or, at best, only loosely radially arranged.

The single most distinguishing feature of the phylum is its water-transport (canal) system. Numerous minute pores called ostia perforate the body wall and allow water to be drawn continuously into a large internal cavity, the spongocoel; the water is then expelled through one or more larger apertures called oscula. This unceasing current is not incidental — it is the sponge's only means of feeding (trapping suspended food particles), of gaseous exchange, and of flushing away nitrogenous waste and excess water. The current is generated and the food trapped by choanocytes (collar cells), flagellated cells lining the spongocoel and canals whose beating flagella draw water inward and whose collar traps particles for intracellular digestion — sponges have no gut, and every digestive event happens inside a single cell. …