AGASSIZ. NATTJEAL HISPTOBT OF THE TOUTED STATES. ill 



those of the JEginida*, some of which are known to undergo a direct 

 metamorphosis."* 



" Considering the mode of reproduction of the Acalephs in 

 genera], the highest Hydroids would, of course, be those in which 

 the medusoid elements prevail, and the lowest, those in which the 

 hydroid elements are most prominent. AVe have, therefore, to 

 inquire, first, whether there are any genuine naked-eyed Medusas 

 which do not originate from Hydra), in order to answer a question 

 already raised respecting the true limits of the order of Hydroids, 

 and the true position of the ^Equorida) and jEginidae. 



" There are iEginida?, unquestionably, which undergo a direct 

 metamorphosis, and it is probable that this is the case with all of 

 them. But are the JEginida? genuine naked-eyed Medusa?, or a low 

 type of Discophora? allied to the Charybdeida? ? My knowledge of 

 this family is too limited to enable me to speak confidently upon that 

 point ; but I am inclined to consider them as belonging rather to the 

 Discophora? proper than to the Hydroids. In the first place, the 

 iEginida? have no radiating chymiferous tubes, as all true naked-eyed 

 Medusa? have ; but instead of them there arise broad, flat pouches 

 from the main cavity, extending towards the margin of the disk, as 

 in Ephyra, the young of Aurelia and Cyanea, and as in the adult of 

 the latter, and of many other genera of Discophora? proper. The 

 iEginida? have no circular chymiferous tube, as all true naked-eyed 

 Medusa? have. Again, the tentacles of the iEginida? are not strictly 

 marginal, and, in the absence of a circular tube, cannot be closely con- 

 nected with it as is the case in all true naked-eyed Medusa?, but are 

 in direct communication with the radiating pouches of the main 

 cavity, as in Pelayia and Cyanea. If, then, for these reasons the 

 yEginidae should be associated with the higher Discophora?, instead of 

 occupying a place among the naked-eyed Medusa^, the importance 

 attached by Gregenbaur to the marginal seam of the umbrella, as a 

 distinctive character of the lower Discophora? would be greatly 

 lessened ; and I rather think rightly so, for many of the higher Dis- 

 cophora?, and among them our common Aurelia, have the margin 

 of their umbrella not only very thin, but turned inward and down- 

 ward as in all Craspedota, and their tentacles arise between indenta- 

 tions of the disc, at some distance from its margin, as is the case in 

 the iEginida?.f 



"As to the iEquorida?, I have no doubt that they are genuine 

 Hydroids, though 1 have not been able to trace with certainty the 

 origin of the Jkquorea of our c^^st to any true Hydroid. But the 

 structure of JEquorea, in its adult Medusa state, is so strictly homo- 

 logous to that of all other naked-eyed Medusa?, that, even if it were 



* P. 108. 



f In his recently discovered genus T«»)o<j<-:, closely allied to Charybda, the pre- 

 sence of a veil is pointed out by Fritz Mullcr. (Halle, Al»h, 18-50.) 



