130 



STRUCTURE OF HYDRA. 



Fig. 71. HYDRA, OR FRESH-WATER 

 POLYPE. 



require separate notice. The group of Hydrozoa, or Hydroid 

 Zoophytes, so named from the little Hydra, or fresh-water 



polype, which may be regarded 

 as its type, will be first described 

 on account of its near con- 

 nexion with the preceding. The 

 Hydra (fig. 71) is a solitary 

 polype, not at all uncommon 

 in ponds or other collections of 

 fresh water, where it is found 

 attached to aquatic plants, or 

 to floating sticks, straws, &c., 

 by means of a kind of sucker 

 at its lower extremity, stretching 

 out its tentacles in search of its 

 food, which consists of minute 

 aquatic worms and insects. These 

 are securely laid hold of by one 

 or more of the tentacles, and are 

 drawn into the mouth, a, which 

 leads to the stomach or general 

 cavity of the body, in which they are digested, and from 

 the walls of which the nutritious portions are absorbed, the 

 portions of the food which are not capable of being digested 

 being cast out through the mouth. 



122. The Hydra multiplies in two ways ; namely, by gem- 

 mation or budding, and by a proper generative process. Little 

 bud-like processes are developed from various parts of the 

 walls of the stomach, which gradually assume the form of the 

 parent, possessing a mouth surrounded by tentacles, and a 

 digestive cavity which is at first in connexion with that 

 of the parent ; the communication is gradually cut off, how- 

 ever, by the closure of the canal of the footstalk of the young 

 polype; and ere long the footstalk itself separates, and the 

 young polype henceforth leads an entirely independent life. 

 Not unfrequently, however, the young polype itself puts forth 

 buds before its separation ; and as many as nineteen young 

 Hydrse, in different stages of development, have been seen to 

 be thus connected with one and the same stock. Another 

 very curious endowment of the Hydra depends upon the same 

 facility of developing the whole structure from any part of it ; 



