18 



BRANCH CCELENTERATA 



CLASS I. HYDROZOA 



In this class are found the worldwide fresh-water Hydras and 

 the marine Hydroid Colonies, such as Campanula' ria or Obe'lia. 



The Hydras are small fresh-water Hydrozo'a from to \ or 

 possibly ^ inch in length. They may be white or colorless, or 

 green or brown. 



The body is a simple cylinder (Fig. 10) or sac, closed at one 

 end, and near the other surrounded by six or eight tentacles, 



Fig. 10. Hydra: Longitudinal section of animal, showing m, mouth; 

 t, tentacle; d, digestive cavity; b, bud; s, spermary; o, ovary; ec, ectoderm; 

 en, endoderm. Magnified. (From Dodge's " General Zoology," American 

 Hook Co., Publishers.) 



above which is the conical hypostome, at the apex of which is the 

 mouth. The muscular fibers of the ectoderm extend lengthwise, 

 while those of the endoderm extend around the body. 1 If 

 disturbed, the Hydras protect themselves by withdrawing into 

 a tiny sphere, while the tentacles contract until they look like 

 so many small buds. The endoderm has flagellate cells lining 

 the gastrovascular cavity. 



1 Hertwig's " Manual of Zoology," Kingsley, p. 230. 



