By Diana R. Kershaw (auth.)

ISBN-10: 041253200X

ISBN-13: 9780412532009

ISBN-10: 940116035X

ISBN-13: 9789401160353

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The nematocysts are discharged from the cnidocytes and can be used for prey capture, dispersal or anchorage. They vary in structure and from a functional standpoint can be divided into three major types: volvent, penetrant and glutinant. Volvents have a simple closed tube which wraps round and entangles the prey. Penetrants have an open tube, which may in addition be armed with barbs or spines, and on discharge the thread penetrates the prey tissues and injects a paralysing toxin. Glutinants are open-ended and sticky and are used for anchorage.

It is thought to exude digestive enzymes which act upon the cytoplasm in its immediate vicinity, the soluble products of digestion being absorbed through the pellicle. Thus unlike the free-living Amoeba and Paramecium it does not form food vacuoles but digests its food outside its body. This method of digestion is common amongst parasitic animals. Eventually the trophozoite grows too large for the sperm morula (the host cell) and then lives in the seminal fluid from which it absorbs nutriment in the same way as do the developing sperms.

The name of the phylum , Porifera, is a reference to these pores which provide for an inflow of water to the paragaster. This water leaves through the osculum. The osculum is so positioned that the excurrent is as widely separated from the incurrent as possible. All sponges are based on this general asconoid plan as outlined above for Leucosolenia. However in its simple form this design imposes an upper size limit and more advanced syconoid and leuconoid sponges show various elaborations of the water current system, achieved through folding of the body wall.

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Animal Diversity by Diana R. Kershaw (auth.)

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