HYDROZOA. 



239 



polpys; (2) Medusae which always produce medusae; (3) Polyps 

 and medusae in alternating generations; (4) Polyps and sessile 

 medusae (sporosacs) united in a polymorphic colony. 



The Hydrozoa are almost exclusively marine. The colonial forms occur 

 mostly on rocky coasts down to a depth of 100 fathoms, but have been 

 found in water 4000 fathoms deep. The medusae belong to the pelagic 

 fauna. For a long time the only fresh-water species known belonged to 

 the cosmopolitan genus Hydra, but more recently both hydroid (Proto- 

 hydra ryderi,* America ; Polypodium hydriforme, Russia) and medusan 

 forms (Limnocodium soivei'byi, Brazil ; Limnocnida tanganyicce, Africa ; 

 Halomises lacustris, Trinidad) have been found. Cordylophora lucustris * 

 occurs in the brackish waters of Europe and America. 



The Hydrozoa may be classified according to characters, derived either 

 from the hydroid or the medusan stage. The former basis gives us four 

 groups : 



(1) Hydraria. Polyps with asexual and sexual reproduction ; no per- 

 sistent colonies, no perisarc, no gonophores (fig. 172). 



(2) Tubulariae. Mostly colonial, with perisarc but without hydrothecae. 

 Reproduction by gonophores (medusae or sporosacs, figs. 91, 175). 



(3) Campanulariae. Colonial, with perisarc and hydrotheca. Repro- 

 duction by gonophores arising in special perisarcal envelopes, the gonotheca 

 (figs. 174, 176). 



(4) Hydrocorallina. Colonial, with massive, calcified perisarc, resem- 

 bling coral. Reproduction by sporosacs or short-lived medusae. 



FIG. 183. American Trachy and Narcomedusae. A, Liriope scutigera. (After Fewkes.) 

 .B, Cunocantha octonaria. (After Brooks.) 



The characters derived from the medusae also give five groups : 



(1) Anthomedusae (Ocellatae). Gonads on the manubrium ; no audi- 

 tory organs ; eyes usually present ; polyp generation present. 



(2) Leptomedusae. Gonads on radial canals ; usually velar auditory 

 organs ; polyp generation present. 



(3) Trachymedusae. Gonads on the radial canals ; tentacular auditory 

 organs ; develop directly to medusae (fig. 183, A.) 



(4) Narcomedusae. Gonads on the manubrium or gastral pouches ; 

 tentacular auditory organs ; no polypoid stage (fig. 183, B.) 



