CHAP, x.] THE FRESH-WATER POLYPES. 99 



of the wide mouth, which is situated 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 

 innutritions 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 is formed ; 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 projections of the surface 

 appear at the bases of the tentacles or nearer the attached 

 end of the body. Within the former (testes) great numbers 

 of minute particles, each moved by a vibratile cilium, are 

 developed and are eventually set free. Functionally, these 

 answer to the antherozooids of plants, and they are termed 

 spermatozoa. 



72 



