520 HABITS OF HYDKA. 



of expanding and contracting within an envelope, that embraces 

 the whole apparatus. When at rest, this vesicle lies at the 

 bottom of the including sac, and the spine is drawn entirely 

 within the latter ; but, when the animal lays hold of any object 

 with one or more of its arms, the vesicle is distended by some 

 unseen means, and protrudes the spine which is seated upon it. 



1168. When in search of prey, the Hydra permits its arms to 

 float loosely through the water. It is rather curious that so 

 inactive a creature as this should principally depend for its food 

 on the minute Crustacea and aquatic Worms, whose rapid move- 

 ments would seem to place them beyond its reach. By lying in 

 wait, however, with its arms thus disposed, the Polype soon 

 succeeds in obtaining its supply ; for if, in their active course, 

 any of these animals should but touch one of the tentacula, its 

 doom is sealed ; it is immediately seized by it ; other arms are 

 soon coiled round it ; and the unfortunate victim is speedily 

 conveyed to the mouth. It has been noticed that, if held for a 

 little time in the arms without being swallowed, soft-bodied 

 animals (such as worms) always die, even when released alive ; 

 whence it has been inferred, with some plausibility, that the 

 spines are the means of conveying into the prey some poisonous 

 secretion, in the same manner as the poison-fang of the Serpent, 

 or the sting of the Bee. Upon minute Crustacea and other hard- 

 shelled animals, however, this secretion appears to have no power. 

 Such animals are often swallowed alive, and their movements 

 within the stomach may often be perceived for some little time ; 

 but, their life being at last destroyed, the process of digestion 

 goes on very rapidly. The transparency of the membrane 

 which composes the stomach, at first permits the outline of the 

 animal to be clearly seen. The film over it gradually becomes 

 turbid, however, and the outline of the animal indistinct ; until, 

 at last, its form is wholly lost. The soft parts are completely 

 dissolved ; and the harder indigestible portions are rejected 

 through the mouth. It would seem that Animal matter is more 

 readily dissolved than particles of Vegetable structure. 



1169. It not unfrequently happens that, in the process o 

 swallowing, the Hydra draws in its own arms, which are coile i 



