Chap. H. GRADATION AMONG ACALEPHS. 119 



those which produce free Medusoe, and others which do not ; some which consist 

 entirely of Hydra^, and othei-s of combined Hydr39 and Medusae; some start from 

 Hydrse, others from Medusa?, — the communities themselves consisting either of a 

 larger number of Hydroids, or of a larger number of Medusa?, when the two types 

 are comljined. These various combinations lead naturally to the formation of 

 subordinate groups among Hydroids. Considering the mode of reproduction of the 

 Acalephs in general, the highest Hydroids would, of course, lie those in which the 

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

 most prominent. We have, therefore, to inquire first whether there are any genuine 

 naked-eyed Medusa? which do not originate from Hydrfe, in order to answer a 

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

 true position of the ^quoridte and -<Eginidte. 



There are iEginidte, vmquestionably, Avhich undergo a direct metamorphosis, and 

 it is probable that this is the case with all of them. But are the ^ginidae 

 genuine naked-eyed Medusa?, or a low type of the Discophorte allied to the 

 CharybdeidfB ? My knowledge of this fimily is too limited to enable me to speak 

 confidently upon that point; but I am inclined to consider them as belonging rather 

 to the Discophorte proper than to the Hydroids. In the first place the ^ginidce 

 have no radiating chymiferous tubes, as all true naked-eyed Meduste have ; but 

 instead of them there arise broad, flat pouches from the main cavity, extending 

 toward 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 Discophorse proper. 

 The .^ginida? have no circular chymiferous tube, as all true naked-eyed Medusas 

 have. Again, the tentacles of the ^ginidte are not strictly marginal, and, in the 

 absence of a circular tube, cannot be closely connected with it as is the case in 

 all true naked-eyed Medusas, but are in direct commimication with the radiating 

 pouches of the main cavity, as in Pelagia and Cyanea. If, then, for these reasons, 

 the ^ginidge shoidd 1?e associated with the higher Discophoroe, instead of occupying 

 a place among the naked-eyed Medusa?, the importance attached by Gegenbaur 

 to the marginal seam of the um)?rella, as a distinctive character of the lower 

 Discophorfe, would be greatly lessened ; and I rather think rightly so, for many 

 of the higher Discophora;, and among them our common Aurelia, have the margin 

 of their umbrella not only very thin, but turned inward and downward as in all 

 Craspedota, and their tentacles arise between indentations of the disc (PI. VII. 

 Figs. 2, 3, and 4; PL VIH. Fig. 5, and PI. IX. Fig. 4), at some distance from its 

 margin, as is the case in the -^gmida?. 



As to the iEquorida^, I have no doubt that they are genuine Hydroids, though 

 I have not been able to trace with certainty the origin of the ./Equorea of 

 our coast to any true Hydroid. But the structure of ^quorea, in its adult 



