THE HYDRA AS A SIMPLE METAZOAN 255 



there are at least two types of nematocysts, each with its special 

 function. One type is hke a grappling organ, since the threads 

 wrap themselves about any fine hair-Hke processes (P'ig. 125 B) 

 and hold fast; the other is able to puncture the outer surface 

 of the prey and drive its everting thread through the surface 

 into the tissues (Fig. 125 A). How this penetration occurs is 

 problematical, but it may perhaps be effected by the extreme 

 rapidity of the eversion, or by means of the barbs at the base of 

 the thread. These leave the capsule pressed together like a triple 

 spear point (Fig. 121 C) and only assume their positions on the 

 everted thread by spreading outward and backward as the eversion 

 proceeds. The poisonous substance of the nematocyst, which is 

 known as hypnotoxin, is seemingly contained within the cavity of 

 the thread itself before eversion and, therefore, covers the outer 



Fig. 125. — Use of nematocysts by hydra. 



A, Insect larva punctured by numeroup nematocysts of the barbed type (redrawn 

 from Jennings, "Behavior of the Lower OrKunisms," printed by permission of the Columbia 

 University Press). B, part of an appendage of a water-flea showing another type of nemato- 

 cyst that seems to function like a grappling hook. (Redrawn from Toppe.) 



surface of the thread when it is finally turned right side out. If 

 the thread penetrates the prey, one can understand that a poison 

 so placed would be highly effective. Sometimes the discharged 

 nematocysts, as seen in the laboratory, show a stainable coagulum 

 about the thread which is the remains of the hypnotoxin. 



The effectiveness of the nematocysts as weapons of offense and 

 defense may be observed if hydras are watched in laboratory 

 aquaria containing water-fleas and other small animals which are 

 their common food. A water-flea blunders against the tentacles 

 of a hydra and is held fast. Ahnost immediately it ceases to 

 struggle, although flickering movements of the appendages may 

 continue for a time. If the hydra has recently fed, the water-flea 

 may soon be released and drop to the bottom, where it may grad- 

 ually resume its activities much as though it were recovering from 

 an anaesthetic. On the other hand, if the hydra has been without 



