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GENERAL ZOOLOGY 



contractile fibers. These fibers run parallel to the long axis of 

 the hydra, so that when they contract, the body of the hydra 

 shortens. The interstitial cells produce egg and sperm cells and 

 also give rise to nematoblasts, which are stinging cells capable of 

 discharging a threadlike tube, the nematocyst, and usually a 



nem 



Fig. 220. — Nematocysts of Hydra before and after discharge, cnc, cnidocil; 

 nem, nematocyst; nu, nucleus of nematoblast; t, threadlike tube. (From Dahlgren 

 and Kepner, Principles of Animal Histology, copyright, The Macmillan Company, 

 after Schneider. By permission.) 



poisonous fluid. Discharge of nematocysts can be produced in 

 a living hydra on a slide in water by the addition of dilute acetic 

 acid or methyl green. The function of the cnidocil, a short 

 projection at the free end of nematocyst, is unknown. Once a 

 thread has been discharged the nematoblast from which it came 

 disappears and a new one takes its place from the interstitial 



