![]() ![]() ![]() Members of the first two groups make up the vast majority of known species, and much of what follows will describe these groups. Living sponges are divided into three classes: the Calcarea, Demospongiae, and Hexa-ctinellida. Sponges play vital ecological roles in many aquatic habitats, especially coral reefs, and because they are thought to be the most primitive living animals, they also play an important role in studies of animal phylogeny. ![]() Despite this simplicity of form and function, sponges are an important component of animal diversity in marine and freshwater habitats (with at least 5,000 living species)-and they have been since they appeared as fossils in the late Pre- cambrian. Sponges are also relatively uniform in manner of life-they are all sessile, aquatic, and feed on particles suspended in water. Sponges contain few different cell types, and these are not organized into the distinct tissues that characterize members of other animal phyla. The body plan of members of the Phylum Porifera, known as sponges, is perhaps the simplest among living animals. ![]()
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