Chap. III. THE GENERA OF CYANEID^. 115 



wide radiating pouches, while in Aurelida; we have branching chymiferous tubes, 

 but also in the sudden thinning of the margin of the disk, which diminishes 

 very gradually in thickness in Aurelidae. The consequence of this is, that while 

 in AurelidEe the disk always expands and contracts uniformly in every direction, 

 in Cyaneidte there is much greater independence in the movements of different 

 segments of the body; some lobes of the umbrella may even be moved separately 

 from the others, no doubt owing to the independent action of the different bundles 

 of the radiating folds of the lower floor. Another result of this peculiar structure 

 is, that the centre of the disk of the Cyaneidte may sink, while the margin is 

 raised, and the whole body assume the form of a broad funnel. 



The Sthenonidse resemble the Cyaneidae already more than the Aurelidae, owing 

 to the great development of their tentacles, and to the fact that their genital 

 pouches hang below the surface of the lower floor. But in this family we have, 

 as in Aurelidai, branching chymiferous tubes, instead of radiating pouches, and the 

 indentations of the margin retain the lobulate character of the young; while the 

 actinostome varies as in Cyaneidae, the arms being more distinct in some genera, 

 and assuming the appearance of flowing curtains in others. 



Of all the Discophorfe, it is to the family of Pelagidse that the Cyaneidae 

 bear the greatest resemblance ; but I do not believe that I have exaggerated the 

 importance of their difference in considering them as distinct. It is true, in Pe- 

 lagida3 the main cavity extends to the periphery in the shape of radiating pouches, 

 as in Cyaneidaj ; but in the Pelagidae these pouches are more uniform, their ter- 

 minal lobes less diversified, and the tentacles arise between the lobes of the margin 

 and not from the lower floor. Again, owing to the greater equality among the 

 pouches, the gelatinous disk thins more uniformly towards the margin, and on that 

 account the disk assumes a more hemispheric shape in its contraction and expan- 

 sion. The genital pouches, also, do not protrude like pendant sacs from the lower 

 •side, and the actinostome, forming a kind of tube before dividing, projects down- 

 ward, and then splits into four distinct, long, waving arms, with thin margins. 



SECTION VII. 



THE GENUS CYANEA COMPARED WITH OTHER GENERA. 



Notwithstanding the fulness of the description of Cyanea arctica which has 

 been presented in preceding sections, I deem it important to call attention once 

 more to those peculiarities of structure of that Acaleph, Avhich, in my estimation. 



