GENERAL ZOOLOGY 



Cnidoblast 



Stylet 



Fig. 10.4. The four t\pes of nematocysts char- 

 acteristic of ail the hydras, shown undischarged 

 and discharged. A and .-f,, desmoneme; B and 

 5,, atrichous isorhiza; C and C,, holotrichous 

 isorhiza, with spiny tube; D and Z),, stenotele, the 

 largest and most potent type. (Redrawn from L. H. 

 Hyman, The Invertebrates: Protozoa through Ctenophora, 

 copyright 1940 by McGraw-Hill Book Co., Inc., 

 printed b\- permission.) 



glass rod, or with a piece of blotting paper soaked in any of a variety of 

 animal fluids, however, evokes immediate discharge. This evidence suggests 

 that the diffusion of substances emanating from the prey in some way sensi- 

 tizes the cnidoblast so that subsequent mechanical contact with the prey 

 induces eversion of the thread. In contrast to the reactions of the cnidoblasts 

 as independent effectors, the muscular responses of the tentacles in the capture 

 of food appear to be coordinated by nerve cells, numerous in the epidermis 

 and present in smaller numbers in the gastrodermis. 



Among coelenterates in general, 17 different types of nematocysts have been 

 recognized, and it has been found that the characteristics of these structures 

 are useful in the classification of coelenterates. Some groups have only one 

 type, and some types are found in only one or a few related groups. The 



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