REPORT ON THE SIPHONOPHOR^. J> 



the former and each of the latter possess on the middle of their upper surface an 

 external aperture (the stigma of the exumbrella). Only in the possession of this 

 hydrostatic apparatus is this Disconula distinguished from an ordinary eight-rayed 

 entirely regular Hydromedusa. Certain Trachoniedusae (Trachynemidae, Pectyllidae) 

 present a most striking resemblance. Even in another further advanced larval stage, 

 which we may call a Porpula, the regular structure of a simple Craspedote person is 

 still retained. The Porpula is distinguished from the Disconula in this, that the 

 number of marginal tentacles has increased from eight to sixteen, and several concentric 

 annular air-chambers ' are apposed to the primary circle of eight. In a still later stage 

 the number of concentric air-chambers is greatly increased, and the tentacles form 

 several rows at the margin of the umbrella. Then between the margin of the umbrella 

 and the central siphon, eight or sixteen conical buds grow out from the subumbrella — the 

 subsequent bearers of the gonophores. They remain closed in the monogastric Discalidae, 

 but acquire a mouth opening in the polygastric Porpitidte and Velellidae. The youngest 

 larvae of the Velellidae are hardly to be distinguished from those of the Porpitidae ; 

 the difference between them only becomes distinct when in the former the 

 vertical skin-fold of the exumbrella begins to be developed (Rataria), in the base of 

 which is formed the diagonal crest of the pneumatophore which determines the 

 amphithect ground-form of the Velellidae. Their youngest larvae, however, are as 

 octoradial as those of the regular Porpitidae. At an early stage they are distinguished 

 by this, that in the Porpitidse each of the eight radial air-chambers forms a stigma, in 

 the Velellidae only two lying obliquely opposed. The resemblance between these 

 larvae and the Pectyllidae suggests that the Disconanths have been originally developed 

 from this group of Craspedotae. If the eight sac-shaped gonads, hanging down from 

 the subumbrella of the Pectyllidae, were not themselves reproductive organs, but 

 medusoid gonophores, and if above these in the gelatinous substance of the disc, a 

 glandular depression or exumbrellar invagination led to the constriction of an air-sac, 

 then from a Pectyllid or Trachynemid there would be derived a very simple Discalid. 



SIPHONULA LARVA op the SIPHONANTH.E. 



Entirely different from the first stage in the development of the Disconanthae, is 

 that of the second legion — the Siphonanthae (Calycophoridae. Physophoridas, Pneumato- 

 phoridae, Aurophoridae). Different as are the Siphonophorae of this manifold legion in 

 their fully developed form, their primary larval forms are very similar, at least so far as 

 their ontogenetic history is yet known. The Siphonula — or the primary medusiform 

 larva — is in this legion from the first not octoradial and regular, but bilaterally sym- 

 metrical. The original circle of tentacles on the umbrellar margin has disappeared ; in 

 all cases only a single capturing filament (Fangfaden) persists, the primary unilateral 



