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Sponges
Sponges are the most primitive of the multicelled animals. They lack organs, but have several different cell types and are built around a system of water canals; water is pulled through these canals and filtered for food. Small structures called spicules provide support for the body, and classification is based on the material and structure of the spicules. Almost all sponges are marine. Five thousand species of sponges have been identified, and there may be five thousand more. The three classes of sponges are the Calcarea (glass sponges), Hexactinellida (silicious sponges), and Demospongiae. Canada contains about five hundred species of sponges.
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The
earliest sponge fossils date back to the Vendian, about 600 million years
ago. They have often been important reef-builders, at many times as important
as all other reef-building organisms combined, but today they only play
a minor part in the construction of reefs. Sponge diversity reached a
high point in the Cretaceous, when dinosaurs reached their largest size.
