DIVISIONS OF THE HYDROZOA. 



59 



also contains numerous green granules, apparently identical with 

 " chlorophyll," or the green colouring-matter of plants. At the base 

 or proximal extremity of the cylindrical liody is a kind of disc- 

 shaped sucker or hydrorhiza, by means of which the animal can 

 attach itself at will to any foreign body. Its favourite position ap- 

 pears to be that of hanging 

 head-downwards, suspend- 

 ed from the stem of some 

 water-plant. It is not, how- 

 ever, permanently fixed, 

 but it can detach itself and 

 change its place at will. 

 At the opposite or distal 

 extremity of the body is 

 placed the aperture of the 

 mouth, surrounded by a 

 circle of from five to fifteen 

 small tubular filaments, 

 which are termed the 

 " tentacles " (fig. 30, a). 

 Each tentacle consists of 

 a tubular prolongation of 

 both ectoderm and endo- 

 derm, and encloses a canal 

 which opens at its base 

 into the general cavity of 

 the body. The ectoderm 

 is richly furnished with 

 thread - cells, which are 

 especially abundant in the 

 tentacles. On the inside 

 of the ectoderm, between 

 it and the endoderm, exists 

 also a peculiar fibrous layer, 

 which has been generally 

 regarded as partly of a ner- 

 vous and partly of a muscular character. The tentacles exhibit the 

 most extraordinary contractility, being capable of retraction till they 

 appear as nothing more than so many little warts or tubercles, and of 

 being extended to a length which is in some species many times longer 

 than the body itself. They are the organs by means of which the 

 Hydra obtains its food, consisting chiefly of minute aquatic organ- 

 isms, such as small worms, insects, Crustacea and Rotifera. These are 

 seized by the tentacles and gradually drawn into the mouth ; but in 



Fig. 30. — The Green Fresh-water Polype {Hydra 

 vit-ldls), suiipeuded head-downwards from a piece 

 of a stem of an aquatic plant, enlarged, a One 

 of the tentacles ; b Testis or sperniariuin, with 

 spermatozoa in its interior; c A single large 

 ovum, protruding from the side of the body ; d 

 Disc of attachment (''hydrorhiza "). 



