I 12 



MANUAL OF ZOOLOGY. 



&i the Sertularida, the Campanularida, the Thecomedusce, 

 and the Medusidce. 



ORDER I. HYDRIDA (Eleutheroblastica, Allman ; Gymnochroa, 

 Hincks). This order comprises those Hydrozoa whose " hydro- 

 soma " consists of a single locomotive polypite, with tentacles and 

 tl hydrorhiza" and with reproductive organs which appear as 

 simple external processes of the body -wall. The hydrorhiza is 

 discoid, and no hard cuticular layer is at any time developed. 



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

 including the various species of " Fresh-water Polypes," as they 



are often called. The 

 common Hydra (fig. 

 37, A) is found abun- 

 dantly in this country, 

 and consists of a tu- 

 bular cylindrical body, 

 the " proximal " extrem- 

 ity of which is expanded 

 into an adherent disc 

 or foot the "hydro- 

 rhiza" by means of 

 which the animal can 

 attach itself to some 

 foreign body. It pos- 

 sesses, 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 tenta- 

 cles, which arise a little 

 distance below the mar- 

 gin 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 (fig. 38), but being extremely long and 



* If the Protohydra of Greeff be a mature form, it also belongs to this 

 order. It differs from Hydra in having no tentacles, but it seems more 

 probably to be the larva of some other Hydroid. 



Fig. 38. The Green Fresh-water Polype {Hydra 

 viridis), suspended head-downwards from a piece 

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

 the tentacles ; b Testis or spermarium, with sper- 

 matozoa in its interior; c A single large ovum, 

 protruding from the side of the body ; d Disc of 

 attachment (" hydrorhiza"). 



