The Porifera are a phylum of sessile aquatic animals with a unique method of filter feeding involving flow of water through a canalsystem, propelled by random beating of flagella occurring singly on cells called choanocytes since a circlet or collar of microvilli surrounds each flagellum. The bigger exhalant openings (oscules) and superficial canals frequently are characteristic features of sponges. Apart from the canal system there are no fixed organs or other multicellular tissues. Sponge cells are relatively free and multifunctional to a degree. Sponge individuals are recognizable as a mass of cells, canals and mineral skeletal structures surrounded by a common layer of flattened cells, the pinacoderm. Sponges, both individuals and different taxa, come in a variety of shapes, colours and sizes and their consistency may range from extremely soft and delicate to rock hard.
There are three Recent classes of sponges which may be recognized on sight with a little experience.
The smallest class is the Calcarea, which as the name implies, has exclusively calcareous skeletal material. The calcareous sponges are predominantly whitish and small, and of fragile consistency.
The second, also small, class is the Hexactinellida, which have siliceous skeletons built of hexaradiate spicules. Often referred to as "glass sponges" Hexactinellida occur predominantly in deep oceanic habitats.
The third and by far the most diverse class are the Demospongiae. Their skeletons are built of siliceous spicules of various forms (but not hexaradiate), often cemented together with great quantities of a keratinous protein called spongin.
Source: Hartman, 1982