Definition: Sessile metazoans possessing inhalant and exhalant pores connected by chambers lined by choanocytes. Water is inhaled through small pores (10-100 µm in diameter), traverses the afferent canals towards the choanocyte chamber and is expelled through efferent canals and the larger exhalant osculum. Water currents are maintained by the concerted action of the choanocyte flagella. Food particles and oxygen are removed from the water by various cells, including the choanocytes. Next to choanocytes there are also at least archaeocytes, which are omnipotent and may evolve into several other cell types. Cells, other than those lining the outside and the inner canals and chambers, move freely in an acellular matrix between the canals and chambers, not fixed in tissues. Firmness of the sponge body is obtained from collagen fibrils in the acellular matrix and from varied mineral supporting material of either calcareous (CaCO3) or SiO2. Sponges have free-swimming larvae.
According to the Systema Porifera (Hooper & van Soest, 2002) there are two subphyla, Subphylum Symplasma and Subphylum Cellularia, and three classes Class Hexactinellida, Class Demospongiae and Class Calcarea.
The classification of the level of subphylum, class and subclass is currently in a state of flux: based on molecular studies, the classes Demospongiae and Hexactinellida are alternatively united in the subphylum Silicea (possess silicious spicules) and Calcispongiae (calcareous spicules), whereas the traditionally recognized demosponge subclasses Tetractinomorpha and Ceractinomorpha are no longer considered monophyletic. There is as yet no consistent alternative higher taxa classification, so we remain with the Systema Porifera until that time.