Chai'. V. THE RHIZOSTOMEiE IN GENERAL. 135 



published of this genus. In Cephea projjer there are four genital sacs and lour 

 oral arches dividing into eight amis, alternating two and two with the genital sacs, 

 as in Rhizostoma. But the position of the eye cannot be ascertained from the 

 figures of Forskal. 



Having thus far analyzed the actinostome of the Rhizostoniida^, with the view 

 of ascertaining the nature of its dift'erent structural elements and its relations to 

 the other parts of the lower floor and of the margin, it may not be out of place 

 here to show, that what has been called the peduncle or proboscis in Acalephs, 

 is a central prolongation on the lower side of the animal, composed of very hete- 

 rogeneous elements in dift'erent families of Acalephs : in Geryonia and allied genera, 

 it is a tube, formed by the prolongation of the lower floor, into which a conical 

 central prolongation of the gelatinous disk extends like a prop. Nothing of the 

 kind exists in any of the Uiscophora) proper, though we have something similar, 

 morphologically speaking, in the bulging of the lower surface of the gelatinous 

 disk in iEquorea, and still more so in Tima and allied genera. In Sarsia, on the 

 contrary, the proboscis consists only of a prolongation of the lower floor, without any 

 corresponding pyramid from the gelatinous disk ; but the tubular proboscis of Sarsia 

 has none of those thickenings of the walls, near its base, which characterize the 

 jieduncle of the Discophorna proper. We have already seen that in Rhizostoma 

 the peduncle is formed by four pillars, which alternate with the genital sacs 

 and, dividing again below their junction, branch to from the eight arms, and thiit 

 in Cassiopea the space intervening between these arms forms a central disk, raised 

 above the surrounding parts of the lower floor, and from the margin of which arise 

 the radiating arms. In these Discophora3 there is no central aperture leading into 

 the main cavity, owing to the close union of the margins of the arms which form 

 the disk. In Salamis there is a similar central disk, from the margin of which 

 the branching arms radiate; but if the figui'c of Quoy and Gaimard can be depended 

 upon, there is a central opening in that disk, as there is, also, in the genus Ilomo- 

 pneusis, figured by Lesson as a Mollusc, though it is, however, unquestionably, an 

 Acaleph, closely allied to the genus Salamis, founded by Lesson upon the Orythia 

 incolor of Quoy and Gaimard. In Favonia and Limnoroa, finally, the centre of 

 the actinostome is developed in another way. Between the pendant arms hangs 

 a kind of central peduncle, which can hardly be compared to that of Geryonia, 

 since it is surrounded by branching arms. Judging from the figures of LeSueur, 

 this peduncle is probably homologous to the central disk of Cassiopea, forming 

 a proboscis-like central prolongation between the arms, instead of a flat disk. But 

 it remains to be ascertained whether that peduncle is solid or hollow, or whether, 

 after all, it is not simply a prolongation of the gelatinous disk projecting beyond 

 the arms. The travelling naturalists who have studied these Medusa) have given 



