Sponges (Encyclopedia of Science)
Sponges are primitive multicellular animals that live in water. All adult sponges are sessile (fixed to one spot), most being attached to hard surfaces such as rocks, corals, or shells. More than 4,500 living species are known. Although some species occur in freshwater, the vast majority are marine, living mainly in shallow tropical waters. Sponges have an amazing power of regeneration: they are capable of growing into a new individual from even the tiniest fragment of the original body.
Sponges vary widely in shape and composition. Some are tall, extending far into the water. Others are low and spread out over a surface. Some have branchlike forms while others appear like intricately formed latticework. Many others are goblet shaped. Despite their differing appearances, all sponges have a definite skeleton that provides a framework that supports the animal. In some species, this skeleton is made up of a complex arrangement of spicules, which are spiny strengthening rods with a crystalline appearance. The soft spongy material that makes up the skeleton of many species of sponges is known as spongin. The fibrous meshwork of this material makes it ideal for holding water.
Most sponges consist of an outer wall dotted with many pores or openings of different sizes. These allow the free passage of water into the central part of the body, the atrium or spongocoel (pronounced...
(The entire section is 510 words.)
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