I trust I have succeeded in showing that the three Orders of the Acalephs are, like the five Orders of the Echinoderms, different degrees of complication of the same structure. In the Hydroids, the organization does not rise above the simple digestive cavity inclosed by the double body-wall; and we might not suspect their relation to the Acalephs, did we not see the Jelly-Fish born from the Hydroid stock. In the Hydroid-Medusae and Discophorae, instead of a simple digestive sac, as in the Hydroids, we have a cavity sending off tubes toward the periphery, which ramify more or less in their course. Now whether there are four tubes or eight, whether they ramify extensively or not, whether there are more or less complicated appendages around the margin or the mouth, makes no difference in the essential structure of these bodies. They are all disk-like in outline, they all have tentacles hanging from the margin, and a central cavity from which tubes diverge that divide the body into a certain number of portions, bearing in all the same relation to each other and to the central cavity. In the Ctenophorae, another complication of structure is introduced in the combination of vertical with horizontal tubes and the external appendages accompanying them. But, whatever their differences may be, a very slight effort of the imagination only is needed to transform any one of these forms into any other. Reverse the position of any simple Hydra, so that the tentacles hang down from the margin, and let four tubes radiate from the central cavity to the periphery, and we have the lowest form of Jelly-Fish. Expand the cup of the Hydra to form a gelatinous disk, increase the number of tubes, complicate their ramifications, let eyes be developed along the margin, add some external appendages, and we have the Discophore. Elongate the disk in order to give the body an oval form, diminish the number of main tubes, and let them give off vertical as well as horizontal branches, and we have the Ctenophore.