Chap. I. SUB-ORDERS OF THE DISCOPHOR.E. 9 



ouglit to be characterized by some sjiecial complication of their structure which does 

 not affect their whole oi'gauization ; or, in other words, they are likely to be sub- 

 orders. Now, such groups unquestionably exist ; and if we compare the structural 

 peculiarities w^hich distinguish the numerous Discophorte allied to Aurelia, Pelagia, 

 and Cyanea on one side fronr those allied to Rhizostoma, Cephea, and Cassiopea 

 on the other side, we cannot foil to perceive that these structural peculiarities do 

 not embrace their whole organization, but only the appendages around the mouth 

 and those of the margin of the disk. And while all the fomilies allied to Aurelia 

 have marginal tentacles and a mouth opening freely, though surrounded by more 

 or less extensive appendages, all the families allied to Rhizostoma are deprived of 

 marginal tentacles, and the appendages of the mouth are soldered along their margin 

 so as to leave only at intervals narrow passages for the admission of the food. We 

 have thus two distinct sub-orders among the Discophora^, for which I would propose 

 the names of Discophor^ SEM^osTOMEiE and Discophor.e Rhizostome^e ; and to these a 

 third sub-order must be added, which I would call Discophor.e Haplostome.e, including 

 the CharybdeidiV! and the iEginidaj. A comparison of the latter with the other 

 naked-eyed Medusas, with which they have generally been associated, will readily 

 show how much they differ from them. Instead of simple radiating tubes communi- 

 cating freely with a circular tube, they have wide radiating pouches so similar to 

 those of the Ephyra3, about the time the tentacles are beginning to form, that the 

 affinity is unmistakable. Moreover, as far as their mode of reproduction is known, 

 the ^Eginida3 agree in their development with the Discophoras Semaeostomete which, 

 like Pelagia, undergo a direct metamorphosis without intervening strobila-like seg- 

 mentation. But they constitute a distinct sub-order inferior to the Rhizostomeoe 

 and SemECostoinejB, inasmuch as the mouth is as simjjle as that of the naked-eyed 

 Medusai ; and the marginal organs, the tentacks and the eye-specks, are also of an 

 inferior character. If these views are correct, the Discophorce should then he 

 subdivided into the following natural sub-orders : — 



I. RHIZOSTOME^. 



II. SEM.EOSTOME.E. 



m. HAPLOSTOME^i;. 



I shall hereafter, I think, succeed in showing that the minor subdivisions of the 

 Discophorse mentioned above are natural families founded upon such peculiarities 

 of structure as determine the form only; while the three sub-orders just mentioned 

 are founded upon complications of structure limited to some of their parts only. 



VOL. IV. 2 



