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The simplest of all multi-celled organisms. Their cells are not organized into tissues, so that some sponges can reconstitute themselves if they are strained through sieves. Most of the 5,000 species are marine and live attached to the seabed, although some bore into and infest the shells of molluscs or the fabric of coral reefs. They are filter-feeders, drawing water into their bodies through tiny pores in the body wall, and expelling it through a central exhalent siphon. Microscopic whiplike flagella power the flow of water. Each day a sponge can process a volume of water that is up to 20,000 times its own body volume and extract from it 90% of the bacteria suspended in it.

There are three major groups of sponges: glass sponges (500 species), which have skeletons of siliceous spicules and mostly live in the deep ocean; calcareous sponges (about 500 species), which have spicules of calcium carbonate and most live in shallow seas; and demosponges (about 4,000 species), whose skeletons are either siliceous or fibrous and which occur in both shallow and deep water. They include the bath sponge (Spongia), one of the few commercially important sponges that are collected by free divers diving to astonishing depths.

M. V. Angel

Subjects: Maritime History — Science and Mathematics.

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