486 NUTRITION AND REPRODUCTION OF HYDRA. 



stances, seems to be given to them as a compensation for their 

 low degree of organisation. While the want of cilia on their 

 tentacula prevents the creation of currents for the purpose of 

 bringing a constant supply of food to the mouth, and thus affords 

 less choice to the animal, the body is so constructed as to be 

 capable of accommodating itself to a prey of extremely variable 

 size ; and the digestive secretion can act upon almost any kind of 

 organised substance, so as to convert it into alimentary materials. 

 And, in like manner, the absence of any special means of aerat- 

 ing the fluids is compensated by the exposure of every part of 

 the tissue, both by its internal and external surface, to the 

 surrounding element. 



1048. The reproduction of the Hydra usually takes place by 

 means of buds developed from its external surface. At first these 

 appear as slight protuberances from the body ; they gradually 

 increase in size, and present somewhat of the form of the parent ; 

 an aperture is then seen at the unattached extremity, and tenta- 

 cula sprout around it. During the whole of this period, the 

 interior of the young Polype communicates with the general 

 cavity of the parent. At first its nutriment is supplied entirely 

 by the latter ; but when the tentacula are developed, it catches 

 prey for itself with much eagerness. It is not an unusual thing 

 to see the young one and its parent struggling for the same 

 worm, and gorging opposite ends of it together. There is still a 

 communication between the stomachs of the two, as appears from 

 the distention of either when the other is fed. As the young 

 Polype advances towards maturity, however, this aperture con- 

 tracts, and is at last obliterated. The stalk, by which the bud 

 is attached, gradually becomes more slender ; and at last it is 

 broken by any slight effort on the part of either animal, and the 

 young one swims off. Not unfrequently, however, it has begun 

 to produce buds from itself, before its separation from its parent ; 

 and thus three generations may be seen united together. During 

 warm weather, this multiplication goes on with great rapidity, 

 if the animals are well supplied with food. From one parent, 

 six or seven buds have been seen to sprout at one time ; and, 

 several of these bearing another generation, as many as eighteen 



