98 DISCOPHOR^. Pakt III. 



pheric part, resembles exactly the ambulacral pouches, but its actinal termination 

 is on the margin of the genital pouches, while the actinal part of the ambulacral 

 pouches communicates directly with the main central cavity. The tentacular or 

 broad pouches communicate also with the genital pouches, and in this respect they 

 stand in the same relation to the main cavity, as the middle interamljulacral pouch ; 

 thus disclosing their interambulacral nature. 



An attempt to designate the radiating segments of the gelatinous disk, in accord- 

 ance with their homological relations, presents great difficulties, owing to the fact 

 that these segments do not correspond to the circumscription of either the ambu- 

 lacral or interambulacral areas of the actinal system. On the contrary, in com- 

 paring the description of the disk with what has just been said of the essential 

 elements of the structure of Cyanea, it appears that of the eight long junctions, 

 four correspond to the middle of the ambulacral system, and four to the middle 

 of the interambulacral system; while the eight short junctions correspond to the 

 middle of the eight large pouches, which are themselves the equivalent of the 

 eight simple radiating tubes without eyes in Aurelia; so that each of the four 

 ambulacral systems corresponds to portions only of the adjoining segments along 

 four long junctions, while the four interambulacral systems correspond to two entire 

 adjoining segments along the long junctions, in the direction of the interambulacral 

 eyes, plus that portion of the other segments which is not covered by the ambu- 

 lacral systems. In this disagreement between the segments of the disk, and the 

 main cavities of the body, we have a new evidence that the disk itself does not 

 belong to the same organic S3^stem as the radiating pouches. In fact, these seg- 

 ments may be homologized with the rows of plates in the calyx of those Crinoids 

 in which these rows do not coincide with the arms or ambulacra, and, from this 

 homology, I infer that the disk of our Medusae is as truly an abactinal structure 

 as the calyx of the Crinoids. 



As in all Discophoraj, the substance of the disk is a gelatinous mass, consisting 

 of immense cells, the caudate prolongations of which traverse it in different direc- 

 tions, assuming the appearance of flat muscular fibres. But this appearance is 

 deceptive, and the substance of the disk does not, in reality, contain distinct mus- 

 cles, though it is highly contractile, especially in the thinner part of the margin. 

 Its movements are owing to the structure of the lower floor. 



The amount of water contained in the tissue of the disk is truly extraordinary. 

 A specimen, weighing thirty-five pounds, exposed to evaporation, left a viscous mass, 

 chiefly composed of common salt, showing the water to be common sea-water. 

 The salt having been washed out with fresh-water, and the organic substance dried 

 simply in the sun, weighed less than an ounce. 



Returning now to the lower floor, and leaving out of consideration all the organs 



