![]() ![]() They also have important roles in feeding and in clearing debris that block the ostia. Archaeocytes (or amoebocytes) are amoeba-like cells that are totipotent, in other words each is capable of transformation into any other type of cell. "Grey cells" act as sponges' equivalent of an immune system. Myocytes ("muscle cells") conduct signals and cause parts of the animal to contract. In addition to or instead of sclerocytes, demosponges have spongocytes that secrete a form of collagen that polymerizes into spongin, a thick fibrous material that stiffens the mesohyl. Sclerocytes secrete the mineralized spicules ("little spines") that form the skeletons of many sponges and in some species provide some defense against predators. Oocytes and spermatocytes are reproductive cells. Rhabdiferous cells secrete polysaccharides that also form part of the mesohyl. Collencytes are another type of collagen-producing cell. Lophocytes are amoeba-like cells that move slowly through the mesohyl and secrete collagen fibres. Other types of cell live and move within the mesohyl: ![]() Pinacocytes, plate-like cells, form a single-layered external skin over all other parts of the mesohyl that are not covered by choanocytes, and the pinacocytes also digest food particles that are too large to enter the ostia, while those at the base of the animal are responsible for anchoring it. All sponges have ostia, channels leading to the interior through the mesohyl, and in most sponges these are controlled by tube-like porocytes that form closable inlet valves. The wave-like motion of the whip-like flagella drives water through the sponge's body. The inner surface is covered with choanocytes, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. A sponge's body is hollow and is held in shape by the mesohyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. ![]()
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