REPORT ON THE SIPHONOPHOR^E. 13 



of the Siphonanthse lies excentrically beside the apex. It arises in the former, indeed, 

 at the place where some Medusae (e.g., Aglaura) form a suctorial organ of attachment. 

 Only subsequently does this apical air-gland become greatly extended, to occupy the 

 greater part of the original larval umbrella, so that one may in a certain sense speak 

 of an " invagination." At any rate, the " air-sac " is originally a portion of the ectodermic 

 epithelium of the exumbrclla, not of the subumbrella. The whole pneumatophore is not 

 a secondary medusoid person, but the modified disc of the primary medusoid larva. 



The many-chambered pneumatophore of the Disconanthas is comparable in its first 

 beginning alone with the single-chambered pneumatophore of the Siphonanthse ; there 

 again it is at first nothing more than an apical gas-gland, which occupies only a small 

 area on the apex of the primary larval umbrella. This simple central chamber becomes 

 rapidly surrounded, however, with a girdle of eight radial chambers, and around these 

 again many concentric rings of chambers are apposed. Thus arises the characteristic 

 octoradial, concentrically chambered air-disc of the Porpitidse, of which the amphithect 

 pneumatocyst of the Velellidse represents only a secondary modification. Sometimes its 

 margin is split into eight or sixteen radial lappets. On the upper surface the disc opens 

 by numerous stigmata, of which the central one alone can be compared to the simple 

 apical stigma of the Pneurnatophoridae. From the lower surface proceed numerous 

 small air-tubules or tracheae, which enter into the so-called "liver" or glandular central 

 organ. I regard only the entodermic canal network of this voluminous "central organ" 

 as " liver " (and perhaps " kidney "), and the compact exodermic parenchyma as a 

 strongly developed gas-gland. The lower ends of the tracheae are not closed, as is 

 generally supposed, but open. They take off the secreted gas, and their terminal 

 apertures correspond to the " funnel aperture of the air-flask " in the Siphonanthse. 

 On the other hand, the stigmata of the upper surface (or of the exumbrella) in the 

 Disconanthae do not serve for the reception of atmospheric air from outside (as is 

 generally supposed), but solely for the exit of the secreted gases, and correspond to the 

 simple apical pore of the Cystonectse. 



AUROPHORE or AIR-BELL. 



The new deep-sea famUies Stephalidae and Rhodalidse are distinguished from all the 

 other Siphonophorae by the possession of a peculiar organ, belonging to the nectosome, 

 designated the aarophore. Since they are in other respects peculiarly organised, they 

 may be regarded as representing a special order, Auronectse. The aurophore is a 

 voluminous gas-gland, which has developed out of a medusiform swimming-bell. It 

 lies below the large pneumatophore, in the dorsal middle line of the stem, opposite the 

 ventral row of buds. The disc-cavity of the swimming-bell has been modified into a 



