The beating of the choanocytes’ flagella creates the sponge’s water current. The collars are composed of microvilli and are used to filter particles out of the water. Choanocytes (also known as "collar cells") function as the sponge's digestive system, and are remarkably similar to the protistan choanoflagellates.Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered. 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 (29,000 feet) or further. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. With no true tissues ( parazoa), they lack muscles, nerves, and internal organs. Sponges represent the simplest of animals. They are primitive, sessile, mostly marine, water dwelling, filter feeders that pump water through their bodies to filter out particles of food matter. The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera.
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