316 



ESSENTIALS OF ZOOLOGY 



cyst. Chemicals, such as weak iodine, acetic acid, or methyl green, 

 when added to the water, will bring this about. Contact will not. 

 In this reaction the stalk and thread are everted, probably by de- 

 velopment of pressure. This type of nematocj^st produces a hypno- 

 toxin which anesthetizes the animals into which it is discharged. In 

 another form the sac is small, the stalk is barbless, and the thread is 

 elastic; it becomes coiled around the object against which it is dis- 

 charged, and thus impedes locomotion of the victim. 



Cnidocil 



Filament 



Nemotocyst 



Nucleus 3 



Barb 



Stalk -- 



Bag 



Bemq'ins of 

 Cnlaoblasb 



Barbless nematocyst 



Fig. 112. — Nematocysts and their function. A, Cnidoblast containing an undis- 

 charged nematocyst, after Schneider ; B, nematocyst everted and extended but still 

 held in the cnidoblast, after Schneider ; C, portion of tentacle, after Jennings : D, In- 

 sect larva attacked by hydra, after Jennings ; E, leg of small aquatic insect with 

 barbless nematocysts on its spines, after Toppe. (Redrawn and modified from 

 Wolcott, Animal Biology, McGraw-Hill Book Company, Inc.) 



The cnidoblasts are produced by a third type of cell, the interstitial 

 cell, which is small and rounded. These are formative cells about the 

 size of the nuclei of the epitheliomuscular cells and quite densely 

 granular in nature. They crowd in between the other cells, especially 

 near their bases. As a nematocyst is discharged, the entire cnido- 

 blast is replaced by an interstitial cell migrating into position. A 

 damaged or spent cell of the body may be replaced from the inter- 

 stitial cell. Besides these three types, there are the scattered, ir- 



