. A manual of elementary zoology . Zoology. 164 MANUAL OF ELEMENTARY ZOOLOGY produced into a long, hollow thread, which lies coiled up in the sac. The space between the thread and the wall of the sac contains a fluid, and there is a specially contractile layer of protoplasm around the sac. The cnidocil is a sense organ. When it is stimulated the contractile layer squeezes the sac, and the pressure upon the contained fluid expels the thread, turning it inside The nematocysts are of three kinds—a large kind with a straight thread provided with barbs at the base, a small kind with a spiral
. A manual of elementary zoology . Zoology. 164 MANUAL OF ELEMENTARY ZOOLOGY produced into a long, hollow thread, which lies coiled up in the sac. The space between the thread and the wall of the sac contains a fluid, and there is a specially contractile layer of protoplasm around the sac. The cnidocil is a sense organ. When it is stimulated the contractile layer squeezes the sac, and the pressure upon the contained fluid expels the thread, turning it inside The nematocysts are of three kinds—a large kind with a straight thread provided with barbs at the base, a small kind with a spiral thread, and a second small kind with a straight thread and a narrower sac than the others. Neither of the small kinds has barbs. The broad end of each cnidoblast is anchored into the body by a process which runs inward towards the structureless lamella. The tentacles are covered with a number of warts, each con- sisting of a large musculo- epithelial cell, in which is embedded a group of cnido- blasts consisting of one or two of the large kind with several of the smaller kinds around them. Each of the kinds of nematocysts has a function of its own. Those of the large, barbed variety are weapons of offence and perhaps also of defence. Their cnidocils are affected by chemical stimuli afforded by the substances given off from the bodies of other animals. When the nematocysts are discharged, their barbs emerge first and make a wound in the tissues of the prey, into which the thread is driven. In piercing the horny skin of the water-fleas, upon which the Hydra principally feeds, they are assisted by the corrosive action of a fluid which they contain, either in the hollow of the tucked-in 1 It is maintained by some authorities that the thread is expelled not by a contractile layer of protoplasm, but by the swelling up of a jelly which (and not a fluid) they believe the nematocyst to FlG. 103.—A transverse section of Hydra, stained and seen under the low power of the mic
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
