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PROCEEDINGS OF THE 



[April, 1857. 



its air-tube, are two-fold, while the tubuliform homologue of the 

 digestive trunk is single, inducing an appearance which leads us 

 to infer something like a partial fission of the single basal disk of 

 Eudoxia. In the same genus the simple digestive trunk of Eu- 

 doxia assumes the elongate tubular form, contains at intervals, 

 groups, each of which have all the principal elements of an Eu- 

 doxia, with the exception of the sexual individuals, which have 

 not yet been observed. In many Diphyidse, the same character 

 of the digestive trunk is preserved, but in Abyla, the bracts disap- 

 pear. In Diphyes and its immediate allies, there appear to be two 

 bell-cavities to the basal medusa-disk, and, except in Prayia, a 

 single air-tube, as if the fission-like modification of Prayia were only 

 in part here preserved. The basal disk, therefore, is among the 

 Diphyidae highly developed, and assumes a variety of extraordinary 

 forms, as in Cuboides. Among the Physophoridse, on the contrary, 

 it is entirely lost, being reduced to the mere parenchyma of the 

 air-vesicle, which serves as the float of the community. Here, 

 however, the swim-bells, which generally constitute the upper- 

 most buds on the tubuliform stem, receive an extraordinary devel- 

 opment in numbers, occupying a very great portion, sometimes 

 almost the wbole of the tubular trunk; a very different condition 

 from the single swim-bell of Eudoxia. They also suffer extraor- 

 dinary modifications of form in such genera as Hippopodias and 

 Apolemia. All the other principal elements, however, of the gem- 

 miferous trunk appear to remain in most genera of this group, 

 such as the lateral digestive trunks or polyps, the bracts, the ten- 

 taculiforrn, and the sexual individuals. I have hitherto spoken 

 (for the purpose of not at once making exceptions in the enunciation 

 of a broad proposition) of the bracts as well as the " polyps" and 

 " tentacula," as distinct individuals; but it is to be remarked, that 

 the bract holds so constant a relation to the "polyp," that it 

 were well worth to examine whether it does not represent, for 

 each one of these digestive trunks, the disk; to which we are also 

 led by a consideration of Prayia, where the helmet-shaped bract 

 holds a similar relation to the digestive trunk as is held by the 

 basal disk in Eudoxia to the analogous part. However this may be, 

 in the next group, that of Physalidas, not only the swim-bells, but 

 the bracts also, entirely disappear, the air-bladder or float assumes 

 an enormous development, and as we have said before, the tubuli- 

 form homologue of the original digestive trunk in Eudoxia, ap- 

 pears to be reduced to a most modified condition, all that we 



