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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 out. 1 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 contain. 



FlG. 103. — A transverse section of 

 Hydra, stained and seen under 

 the low power of the micro- 

 scope. 



ect., Ectoderm; end., endoderm ; st.l., 

 structureless lamella. 



