Chap. II. THE DIFFERENT RADIATA. 43 



in opposite directions, two main stems of the cliymiferous tubes. Each of these 

 divides into two forks, wliich in their turn are subdivided again, so that each stem 

 ends in four In-anches o})ening into four vertical tubes, extending without farther 

 ramifications to the lower margin of the animal ; and all this with only such differ- 

 ences in the number of In-anches as occur between different genera among genuine 

 Meduste. We have even in Idjia, in the two sim^ale tubes that follow the flattened 

 sides, a close resemblance to the arrangement prevailing in Aurelia, where straight, 

 simple tul)es alternate with those that are subdivided. 



These ficts may be sufficient to show that the Ctenophoraj cannot be separated 

 from the ordinary bell-shaped Meduste ; yet Avhen we come to examine the 

 characteristics of the orders in the class of Acaleplis, we shall trace tliose 

 homologies farther, and also show how the structure of all the Ctenoj)liorte, even 

 of those differing most from the type of Idjia, sucli as Cestum, Lesueuria, 

 Bolina, and Pleuroljrachia, is strictly homologous to that of Idyia in all their 

 peculiarities. 



Among the Discophora^ there exists also a great diversity ; and I shall compare 

 closely all their different types, when examining tlic natural limits of that order. 

 Suffice it to say here, that tlie Kliizostomes, which have been represented as widely 

 differing from the others in the structure of their mouth, differ only in so lar that 

 the edges of the four pendent branches of the central peduncle — wliich are free 

 for their whole length in Aurelia, Chrysaora, Pelagia, and Cjanea, and form four 

 channels leading to a central opening, the so-called mouth, that opens into the 

 main cavity — are soldered together for their whole length in Rhizostoma, Cephea, 

 and Cassiopeia, leaving only here and tliere small openings between their folds, 

 through which a less bulky tbod passes in the same way as in the conmiou Medusas, 

 along the channels thus formed, into the main cavity of the body. The homology 

 is perfect, the only diffei'ence being that the edges of these four ajipendages coalesce, 

 instead of remaining open. (See PI. XIII. and XIV.) 



Before the mode of reproduction of the so called naked-eyed Medusa3 was known 

 as it now is, no question could be raised as to their affinity ; and they were simply 

 referred to the order of Discophora^. But since many of them have been ascer- 

 tained to arise from buds formed upon the stem, or between the tentacles, of the 

 crown of the so-called Hydroid Polyps, the question now is, whether their association 

 with the ordinary Discophorte in one and the same order is true to nature or not ; 

 and further, what should be the position in a natural system of the Hyih-oids 

 themselves, which, before these discoveries, were unhesitatingly associated witli the 

 ordinary Polyps. Does this show that genuine Polyps produce genuine Meduste, 

 to be considered as distinct animals ; or that the Hydroids, with their respective 

 Medusoe, are only alternate modes of existence of the same being? Or does it follow 



