Variations Among ScyphomeduscB. 565 



that these organs are large and conspicuous, four in number, and 

 usually the first to attract attention. It is doubtless on this 

 account that so many of the earlier observations concerning 

 variation in these medusae dealt almost exclusively with this 

 feature. 



Among the commonest variation is the hexamerous form, where 

 there are six each of the gastric lobes, gonads, and oral arms. 

 This will be observed at a glance by comparing the several tables, 

 especially Nos. IV and V. Next in frequency is the pentamerous 

 type, where there is a symmetrical arrangement of the organs 

 upon the plan of five. As the several details of these variations 

 are specified so far as their numerical aspects are concerned it is 

 only necessary to refer to the tables already cited. It may be well 

 to notice briefly a few features not capable of tabulation. Among 

 these are the not infrequent occurrence of signs of atrophy, as 

 shown in Figs. 11 and 12. In the former it will be observed that 

 associated with the small size of the pouch and gonad is the entire 

 absence of the interradial canal and its marginal organ. In the 

 latter will be observed the presence of a mere rudiment of a regres- 

 sive gonad in one of the pouches, while in the opposite compound 

 pouch there are two gonads, and in this case the absence of the 

 perradial canal system. 



Associated with variations in the number, is that of variation in 

 the size and relations of the organs, as already pointed out in the 

 figures cited. Attention was directed to the compound character 

 of the organs. This is a very common occurrence, and probably 

 is indicative of the manner of the origin of supernumerary organs 

 of this character. However, the pentamerous and hexamerous 

 condition is frequently distinguishable in the ephyra, and seem 

 to be quite distinct from the beginning And I have found in 

 Cyanea that occasionally trimerous polyps occur, and probably 

 give rise to trimerous ephyrae and later trimerous medusas. It may 

 not be improbable that the suggestion of Ehrenberg {op. cit.), 

 that the circular gonads which he observed were the result of 

 fusion of what may have been earlier distinct organs, is quite as 

 likely as that above. It may be suggested in this connection that 

 I have never seen a case such as that cited by Ehrenberg, though 

 its occurrence does not seem improbable, but in every case which 

 has come under my observation of a compound gastric pouch, the 

 gonads have been more or less distinct, as indicated in Fig. 12. 



