CHAPTER III. 

 THIRD BRANCH OF ANIMALS. 



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HYDROIDS, ETC. (Coelentezata, hollow intestine). 



General Characteristics. A simple sac, as the Hydra, 

 composed of two-cell layers, possessing a stomach, or di- 

 gestive cavity. The mouth is encircled by tentacles, which 

 are hollow, and connect with the stomach. 



Class I. HYDROZOA. 



Order I. Hydroids. One of the commonest animals 

 of the aquarium, if the water is taken from the brook or 

 stream, is the Hydra 

 (Fig. 13) a simple, 

 elongated stomach 

 one quarter of an 

 inch in length, end- 

 ing in a mouth that 

 is surrounded by from 

 five to eight tentacles 

 that are extensions of 

 the body, hollow, and 

 connecting with it. 

 If examined closely, 

 myriads of small cells 



FIG. 13. Fresh-water Hydra viridis. JL. 

 Long-armed Hydra feeding on small ani- 

 mals, a. 2. Hydra fusca throwing off 

 young Hydra-buds. 



will be seen, many of 

 which contain deli- 

 cate threads or darts, called lassoes, that are thrown out 

 as weapons of defense (Fig. 14). By cutting a Hydra 



