HYDRA AND CCELENTERATES IN GENERAL 



121 



cnc 



appears, and then the rest of the tube rapidly turns inside out. 

 Nematocysts are able to penetrate the tissues of other animals, 

 but only at their greatest speed and before eversion is completed. 

 Even the extremely firm chitinous covering of insects may be 

 punctured by these structures (Figs. 55, B and 57, A). The cnido- 

 cil when touched was 

 for a long time sup- 

 posed to cause the 

 explosion of the nema- 

 tocysts, and for this 

 reason is known as a 

 " trigger." One can 

 easily prove, however, 

 that mechanical shocks 

 have no influence upon 

 the nematocysts. In- 

 ternal pressure produced 

 either by distortion or 

 by osmosis, is effective. 

 For this reason chemi- 

 cals which increase the 

 osmotic pressure within 

 the cnidoblast cause the 

 eversion of the thread- 

 like tube (123, 1 08). 

 An animal when " shot" 

 by nematocysts is im- 

 mediately paralyzed, 

 and sometimes killed, 

 by a poison called hyp- 

 notoxin which is in- 

 jected into it by the FlG ' 5 6 - Nematocysts before and after dis- 

 , i charge. I, threadlike tube ; new., nemato- 



' cyst ; cnc., cnidocil ; ??/., nucleus of cni- 



Nematocysts are de- doblast. (From Dahlgren and Kepner 



veloped from interstitial after Schneider.) 



nu. 



/ 



