REPORT ON THE SIPHONOPHOR^E. 27 



(with elliptical or oblong shell, and a vertical crest). Huxley (in 1859) gave the first 

 accurate definition of this group (9, p. 71) : — " Physophoridse without nectocalyces or 

 hydrophyllia ; with short, clavate, simple or branched, submarginal tentacles. A single 

 central principal polypite. Pneumatocyst flattened, divided into chambers by numerous 

 concentric partitions, and occupying almost the whole of the discoidal ccenosarc." The 

 description which Huxley gave of the two genera Velella and P or pita is very accurate, 

 as is also that given in 1853 by Kolliker (4). Another description of both has been 

 recently published by Alexander Agassiz (1883, 57). His opinion is that these Siphono- 

 phorse are more nearly allied to the Tubularise, and on the other side to the Hydro- 

 corallinse (Milleporidse, &c). The oldest figures are those of Forskfd (11). 



All authors hitherto agree in the general opinion that the Disconectse are the most 

 highly developed of all Siphonophorae, and that they form the terminal group in their 

 systematic arrangement, as being most closely allied to the Physalidse. In my opinion 

 this general view is quite erroneous. I am convinced, mainly by their ontogeny, that 

 the Disconectse have preserved the original medusoid structure more than any other 

 Siphonophorse (or the Siphonanthse). They retain the original umbrella with its corona 

 of marginal tentacles, and the original manubrium, which is the " principal polypite," 

 or better, " central siphon." The gonostyles, or the polypiform persons which bear the 

 gonophores (either mouthless palpons or mouth-bearing siphons), bud from the sub- 

 umbrella (or the lower face of the disc) ; while the polymorphous persons in all the 

 other Siphonophorse bud from one side of the manubrium (or the primary siphon) in its 

 ventral line. The primary larva of all the Disconectse (Disconula) is a regular octoradial 

 Medusa, with a marginal corona of eight radial tentacles, widely different from the 

 larva of all other Siphonophorse (the Siphonula, with bilateral umbrella and a single 

 tentacle). I suppose, therefore, that there is no direct relation between the Disconanthse 

 and the Siphonanthse, and that the two subclasses or legions have a different origin : 

 the former arising from Anthoroedusse (Codonidse, Euphysidse), the latter from Tracho- 

 medusse (Trachynemidse, Pectyllidse). 



Nectosome and Siphosome. — The two main parts of the fully-developed body, which 

 we separate as nectosome (or swimming apparatus) and siphosome (or nutritive apparatus), 

 have in the Disconectse .a mutual relation very different from that of the other Siphono- 

 phorse (the Siphonanthse). The nectosome is represented by the exumbrella, or the 

 upper half of the discoidal body, which includes the polythalamous pneumatocyst ; the 

 siphosome, however, is formed by the subumbrella, or the lower half of the horizontal 

 disc, and by the different organs which are attached to its margin and to its lower face. 

 The boundary face between the two main parts is the upper face of the centradenia 

 or the so-called " fiver"; it is in close contact with the lower face of the pneumatosaccus. 

 The constant organs, which depend from the subumbrella, are the following : — (I.) The 

 large central siphon, in the middle of the lower face ; (II.) a simple or multiple corona 



