HYDRA. PHYLUM CCELENTERATA 

 102 n I A^xv 



comnninication, so that food obtained by either is available for the 

 other In spite of this, bud and parent may attempt to swallow the 

 same prey while the parent may snatch food away from the young. 

 In starved individuals buds may be withdrawn. Occasionally a 

 hvdra will reproduce by fission, the whole body dividmg, either 

 lengthwise or transversely, into two. In this event, as m the fission 

 of Paramecium, structural development as well as the growth of 

 each product of fission must take place after separation, whereas 

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

 place before fission. 



REGENERATION 



A property akin to asexual reproduction is that of regeneration 

 or the replacement of lost parts, 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. This ability is connected with the presence 

 of large numbers of the unspecialized interstitial cells. 



OBELIA 



HYDROID COLONIES 



We must now look at the budding of Hydra from a somewhat 

 different point of view. By the outgrowth of buds, the animal 

 increases the size of its body in precisely the same way as 

 Carchesiiim ; 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 Hydra the process is carried further by the fission of 

 the repeated part from the parent body, so that an act of repro- 

 duction takes place, but it is easy to imagine that this might 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. A common example 



