Increased co-ordination between colonial cells appeared with the evolution of the sponges (Porifera). Sponges may be formless lumps on the sea floor reaching two metres in size. Their surfaces are covered with tiny pores through which water is drawn into the body by flagella and then expelled through larger vents. The sponges feed by filtering particles from this stream of water passing through its body. Some sponges produce a soft flexible silica-based substance which supports the whole organism, whereas other sponges secrete lime or silica to create a hard "skeleton" for support. Despite the elaborate skeletons that some sponges are able to produce they cannot be considered as an integrated multi-cellular animals since they have no nervous systems nor muscle fibres. Sponges are primitive, sessile, mostly marine, waterdwelling filter feeders that pump water through their matrix to filter out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs. In some ways they are closer to being cell- colonies than multicellular organisms. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m. Though the fossil record of sponges dates back to the Precambrian era, new species are still commonly discovered. The structure of a sponge is simple: it is shaped like a tube, with one end stuck to a rock or other object and an open end, the osculum, open to the environment. The spongocoel, or interior of the sponge, is composed of walls perforated with microscopic pores that allow water to flow through the spongocoel.
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