HYDRA AND OBELIA. POLYPS AND MEDUSA 173 



body of the parent. Several may be formed at the same 

 time, and a bud may form secondary buds before it is set 

 free. While it is still on the parent, the bud is wholly a 

 part of the body of the latter. Each of the layers of the 

 parent is continuous with the corresponding layer of the 

 bud, a suitable stimulus is transmitted by the nervous 

 system from one to the other, and the entera are in free 

 communication, so that food obtained by either is available 

 for the other. Occasionally a Hydra will reproduce by 

 fission into two, either lengthwise or transversely, of the 

 whole body. In this case, as in the fission of a Paramecium, 

 structural development as well as the growth of each pro- 

 duct of fission must take place after separation, whereas in 

 the bud, as we have seen, the structural development takes 

 - place before fission. 



A property akin to asexual reproduction is that of 



regeneration or the replacement of lost parts, 

 Regeneration. ?. , , * r TT , . , • 1 



which is possessed by Hydra in a very high 



degree. To some extent all organisms have this power, 



but as a rule the higher the animal the less is its faculty for 



regeneration. In man it is little more than the power of 



healing wounds. Not only will Hydra grow anew any part, 



such as a tentacle, which is cut off, but any fragment of the 



body, provided it be not too small and contain portions of 



both layers, will grow into an entire animal. 



We must now look at the budding of Hydra from a some- 

 what different point of view. By the out- 

 colonies. growth of buds, the animal increases the size of 

 its body in precisely the same way as Carchesium ; 

 that is to say by the addition of new members, each of 

 which repeats the whole structure of the body as it existed 

 at first. In the case of Hydra the process is carried further 

 by the fission of the repeated part from the parent body, so 

 that an act of reproduction takes place, but it is easy to 

 imagine a case in which this would not happen. The result 

 would be the permanent conversion of the body of the 

 Hydra into a colony, of which the buds would be the 

 zooids. Now there are a number of animals related to 

 Hydra in which this actually takes place. Such animals 

 are known as hydroids, and nearly all of them are marine. 



