134 



ZOOLOGY. 



thicker titan a sewing thread. It consists of two layers 

 the outer, the ectoderm ; the inner, the en do derm. Its 



base is furnished with 

 a sucker by which it 

 is able to attach itself 

 to some solid object. 

 The attachment, how- 

 ever, is not permanent, 

 as it is able to change 

 its position when it 

 wishes to do so. Its 

 mouth is surrounded by 

 a number of tentacles 

 which it has the power 

 of lengthening or short- 

 ening with great facil- 

 ity. When completely 

 retracted, they have the 

 appearance of little 

 warts of j elly. The ten- 

 tacles are hollow tubes, 

 and are formed by pro- 



FIG. 5G. FRESH-WATER HYDRA (enlarged). 

 It is attached to a fragment of duck- weed, 

 and comprises three individuals, of which 

 the largest has the tentacles more expanded 

 than the others. The black lines represent 

 the digestive tube. The individual to the 

 right has been tied by a thread to show how 

 the animals may be multiplied by division. 

 Below are two detached individuals sup- 

 posed to be formed in this way. 



longations of both the 



ectoderm and endoderm. 

 They communicate at 

 their bases with the 

 cavity of the body. It 

 is very greedy, and 

 any small animal which 

 it seizes with its tenta- 

 cles is soon benumbed by the thread cells with which 

 they are furnished. In the brown hydra the tentacles 

 are sometimes eight inches long. 



These animals feed on minute worms, insects, Kotifera, 

 water-fleas, and other small Crustacea. The mouth 

 leads into a digestive tube, which opens into the general 

 cavity of the body. This cavity contains water, with 

 which the particles of food are mixed. The indigest* 



