70 



MANUAL OP ZOOLOGY. 



ORDER I. HYDRIDA (Gymnochroa, Hincks). This order 

 comprises those Hydrozoa whose ' hydrosoma ' consists of a, 

 single locomotive polypite, with tentacles and ' hydrorhiza,' and 

 with reproductive organs which appear as simple external pro- 

 cesses of the body-wall. The hydrorhiza is discoid, and no hard 

 cwticular layer is at any time developed. 



The order Hydrida comprises a single genus only (Hydra), 

 including the various species of 'Fresh- water Polypes,' as 



a 



Fig. 12. Morphology of Hydrozoa. . Diagrammatic section of Hydra. The 

 dark line is the ectoderm, the fine line and clear space adjacent are the 

 endoderm. 6. Hydra viridis, showing a single ovnm contained 1 in the 

 body wall near the proximal extremity, and two elevations containing 

 spermatozoa near the bases of the tentacles, c. Hydra vulgaris with an 

 nndetached bad. d. Thread-cell of the Hydra, greatly magnified. 



they are often called. The common Hydra (fig. 12 c) is 

 found abundantly in this country, and consists of a tubular 

 cylindrical body, the ' proximal ' extremity of which is ex- 

 panded into an adherent disc, or foot the ' hydrorhiza ' by 

 means of which the animal can attach itself to some foreign 

 body. It possesses, however, the power of detaching the 

 hydrorhiza at will and thus of changing its place. At the 

 opposite, or ' distal,' extremity of the body is placed the 

 mouth, surrounded by a circlet of tentacles, which arise a 

 little distance below the margin of the oral aperture. The 

 tentacles vary in number from five to twelve or more, and 

 they vary considerably in length in different species, being 

 much shorter than the body in the Hydra viridis, but being 

 extremely long and filamentous in Hydra fusca. They are 

 highly extensile and contractile, and serve as organs of pre- 

 hension. Each consists of a prolongation of both ectoderm 

 and endoderm, enclosing a diverticulum of the somatic cavity, 

 and they are abundantly furnished with thread-cells. The 



