CHAP. VI.] THE FRESH-WATER POLYPES. 343 



When any small animal, such as a water-flea, swimming 

 through the water comes in contact with the tentacles, it is 

 grasped, and conveyed by their contraction to the aperture 

 of the wide mouth, which is situated on the summit of a 

 cone (hypostome) in the middle of the circle formed by the 

 bases of the tentacles. It is then taken into a cavity which 

 occupies the whole interior of the body; the nutritive 

 matters which it contains are dissolved out and absorbed 

 by the substance of the Hydra; and the innutritious 

 residuum is eventually cast out by the way it entered. 

 Small pieces of meat, brought within reach of the tentacles, 

 are seized, swallowed and digested in the same manner. 



If a Hydra is well fed, bud-like projections make their 

 appearance upon the outer surface of the body. These 

 gradually elongate and become pear-shaped. At the free 

 end a mouth appears; and around it minute processes are 

 developed and grow into tentacles ; and thus a young Hydra 

 is formed by gemmation from the parent. This young Hydra 

 becomes detached sooner or later, and leads an independent 

 existence; but, not unfrequently, new buds are developed 

 from other parts of the parent before the first is detached, 

 and the progeny may themselves begin to bud before they 

 attain independence. In this manner, temporarily compound 

 organisms may be formed. Experiments have shewn that 

 these animals may be cut into halves or quarters and that 

 each portion will repair its losses, and grow up into a perfect 

 Hydra; and there is reason to believe that this process 

 of fission sometimes occurs naturally. 



The Hydra multiplies by budding through the greater 

 part of the year; but in the summer sexual organs appear 

 in the form of projections of the surface of the body. 

 These, when ripe, may be resolved into a larger and a 

 smaller set. The latter may appear on any part of the 



