• REPORT ON THE SIPHONOPHOBJE. 11 



phore, and underneath that a (simple or multiple) horizontal corona of swimming-bells, 

 and in the middle of the latter (in the dorsal median line of the nectosome, opposite the 

 ventral row of buds) a large aurophore, a new, hitherto unknown organ, which is to be 

 regarded as a swimmino-bell modified into a large gas-gland. 



NECTOPHORES ok SWIMMING-BELLS. 



(Nectocalyces, Kectozooids, Schwimmglucken, Schvrirn mhohlenstiicke.) 



The swimming-bells generally exhibit so clearly the structure of the umbrella of a 

 simple hydromedusa person, that there can be no doubt as to their morphological 

 import as the locomotive organ of such a person — in the sense of the poly-organ theory. 

 The cavity of the gelatinous disc is clad throughout by the circular muscle layer of the 

 subumbrella, and its distal margin protrudes in the form of a true velum, at the base of 

 which runs the circular canal which unites the four radials. The poly-person theory 

 interprets these facts, however, in this way, that each swimming-bell is to be regarded 

 as a medusoid person or " locomotive zooid," in which the locomotor umbrella is 

 well developed, but the nutritive manubrium degenerated. To this the medusome 

 theory has to reply that such a phylogenetic interpretation is not corroborated by the 

 known ontogenetic facts. The protococlon, or the "primary swimming-bell" of the 

 Siphonula is only an organ of this medusoid person. It does not remain as such in 

 most (if not all) of the Calyconectse, but becomes thrown off, and replaced by 

 metacodons or heteromorphie "secondary swimming-bells." In the monogastric 

 Calyconectse the protococlon is modified into the covering bract of the Eudoxia, while 

 in the Physonectse, Cystonectse, and Auronectae it becomes the pneumatophore. The 

 numerous metacodons, which in the. Physonectas and Auronectse, as well as in the 

 Polyphyicke, form the two-tiered, many-tiered, or radially disposed swimming column, 

 are either displaced umbrellas of medusomes, which are repeated metamerically in the 

 several cormidia, or are simply vicarise or "reserve bells" ("Ersatzglocken") which 

 arise by the multiplication of such umbrellas, and are often present in great 

 numbers. 



PNEUMATOPHORE or SWIMMING-BLADDER, 



(Air-Chamber, Lufthammer, Schivimmblase.) 



The hydrostatic organ of the Siphonophorae, which is known as pneumatophore, is 

 only absent in one order, that of the Calyconectse. In the other Siphonophorse it is 

 generally present and that in two different forms; the Physonectse, Cystonectse, and 

 Auronectae possess a simple swimming-bladder at the superior or apical pole of the stem; 



