They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Sponges are either radially symmetrical or asymmetrical. Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes (collar cells). Other characteristics of sponges include a system of pores (also called ostia) and canals, through which water passes. This is the simplest kind of cellular organization found among parazoans. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells.
Sponges are primarily marine, but around 150 species live in fresh water. There are three classes of sponges existing now: Demospongiae, Calcarea and Hexactinellida (other classes have become extinct and are found in the fossil record).Sponges are a diverse group of sometimes common types, with about 5000 species known across the world. Sponges occur in marine and freshwater but do not occur on land. Sponges exist in all different shapes, sizes and colours, and members of the same species can vary depending upon where they live. This passage of water also brings in oxygen and takes out carbon dioxide and wastes. They feed by pumping large quantities of water (hundreds of litres per day) through their bodies and filtering out tiny organisms and dead plant and animal particles as food.
As a result, they are unable to move to escape predators, but discourage attack with strong chemicals and an exterior of bristly textures. Sponges are sessile animals, they live permanently affixed at the base and do not move around as adults. Their skeleton can be composed from two materials: spongin (a soft, flexible organic material - made of the protein collagen) and spicules (small rods of calcium carbonate or silica). They are considered simple animals as they consist of many cell types with special functions and they lack tissues and organs. Porifera means "pore bearer" and is the scientific name for sponges. Photo by Clay Bryce, image copyright WA Museum Collection Highlights | Updated 10 years ago Sarcotragus sp.
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