32 



THE HYDROMEDUSAE 



(Aglantha) or six (Geryonia) short perradial lappets ; in the 

 Petasidae and Trachynemidae it opens through the short manu- 

 brial cavity into the subumbral gastric cavity ; in the Aglauridae 

 and Geryonidae it opens directly into the gastric cavity of the 

 pseudo-manubrium. The perradial canals which lead from the 



gastric cavity are four 



(Liriope) or six (Gery- 

 onia) ; to these inter- 

 radial canals are often 

 added ; both open as 

 usual into a circular 

 canal. ' From the latter, 

 in old siDCcimens of many 

 species, blind centri2Jctal 

 canals grow backwards 

 towards the apex of the 

 bell, but never reach the 

 gastric cavity (Geryonia); 

 their number varies, but 

 may amount to twenty- 

 seven between every two 

 radial canals (Olindias). 



The generative cells are 

 formed on the underside 

 of the radial canals, either 

 in their course through 

 the subuml)rella (Gery- 

 or through the 

 pseudo - m a n u b r i u ra 

 (Aglaura). The cells are arranged in bands, which are flat and 

 do not project on the subumbral surface (Geryonia), or in sacs 

 which dei)end into the bell cavity from the subumbrella (Aglantha), 

 or from the pseudo-manubrium (Aglaura). In the Pectyllidae the 

 sacs are perradial and interradial in position, and are each divided 

 into two, and supi)orted by a lamina which passes across the bell 

 cavity from the manubrium to the radial canals (mesogonia) ; these 

 laminae do not appear to have any relation to the mesenteries of 

 Scyphomedusae. The sense organs are ocelli and tentaculocysts. 

 Ocelli are comparatively rare in this group ; when present they are 

 generally simple pigment specks, and only occasionally possess a 

 lens (Olindias ?). The tcnfacidoci/sts are primarily sui)erficial, four 

 in number, and perradial in position (Petasus). By their displace- 

 ment and by the intercalation of others there come to be, in 

 many cases, eight (Marmanema) or twelve (Geryonia) nearly })er- 

 radial and interradial tentaculocysts, or sixteen adradial (Khopa- 

 loncma). In Olindias there are between one and two hundred of 



Fio. at). 



Diagrammatic vertical section of Cannarina hastota, 

 jKissing on the riglit tlirough the whole length of a 

 perradial canal, and Ion the left through the outspread 

 lobe of an ovary, a lateral extension of a similar canal. 

 I, mesogloea of the disc and pseudo-manubrium ; r, per- 

 radial canal ; rs, its outer, rl, its inner wall ; g, gener- 

 ative cells; /.-, gastric cavity; 2, tongue-like process; h, . v 

 perouium ; c, circular canal ; vk, cartilaginous marginal Onia) 

 ring. (From Lankester, after Gegenbaur.) 



