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AN INTRODUCTION TO ZOOLOGY 



cells, each cell producing one nematocyst. " First a clear space ap 

 pears in an interstitial cell ; this space enlarges, it acquires a defi- 

 nite wall, and its contents stain deeply. Presently it elongates, and 

 one end is produced to form the thread, which at its first appear- 

 ance is everted and coiled round the outside of the sac. After a 

 time the thread is introverted it is not quite clear how and 



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FIG. 57. The action of nematocysts. A, a nematocyst piercing the chitinous 

 covering of an insect ; B, nematocysts holding a small animal by coiling 

 about its spines. (After Toppe in Zool. Anz.} 



