No. 5.] VARIATION AMONG HYDROMEDUSAE. 249 



the bell the adequate marginal space becomes available. And 

 the fact that this seems so variable would appear to warrant the 

 inference that growth occurs at irregular intervals and areas 

 over the body of the medusa. Might not this fact throw soim- 

 light upon the marked unsymmetry of such forms as those 

 shown in Figs. 10 and 11? This suggestion would seem to 

 find some further support in the fact that very young speci- 

 mens appear to be more constant in their symmetry than those 

 more mature. 



Otocysts. 



In formalin specimens there is a degree of opacity induced, 

 especially about the marginal area of the bell, which often 

 renders difficult any satisfactory examination of these sensory 

 bodies. Hence only a limited number of critical determina- 

 tions on this point were made, but these were sufficient to show 

 a degree of variation both in their number and arrangement 

 quite as marked as in that of other organs. 



Normally they should occur in somewhat symmetrical order 

 between the bases of the tentacles. This, however, is rarely 

 the case. There seems about the same variation in their 

 occurrence and relations as in the case of the tentacles, though 

 I was not able to discover that the latter had any determining- 

 influence upon them. In only a few cases have I been able to 

 demonstrate the presence of more than a single otolith in a 

 given cyst, and in no case more than two. On this point, 

 however, the opacity above referred to, and the abundant pig- 

 ment about the bases of the tentacular bulbs were material 

 obstructions to such determinations, and suggest tentative con- 

 clusions. In matter of shape and size these organs present 

 likewise considerable variation. PI. IV, Fig. I, presents the 

 average aspect of form at a, while at b are shown forms not 

 unusual but variant. 



Summary atid Rei'icic. 



1. Variation among Hydromedusae is of wider extent than 

 had been supposed. 



2. Variation is much greater in some genera than in others. 



