96 ELEMENTARY BIOLOGY. [X. 



aperture 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 innutritious residiium 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 

 bqcomes 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 ^ hearer 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. 



The enlargement formed near the attached end of the 



