REPORT ON THE SIPHONOPHOR^. 47 



Sitbumbretta (fig. l). — The inferior or basal face of the umbrella is nearly hemi- 

 spherical. Its centre is occupied by the central siphon (so), and this is surrounded by a 

 corona of eight gonostyles (gs). The equatorial zone of the subumbrella is beset with 

 eight perradial tentacles, and between these proceed outside the eight interradial lobes of 

 the limbus. 



Pneumatocyst (fig. 2, pf, from above ; figs. 4, 5 iu meridional section). — The central 

 air-chamber {p>h) is spheroidal, large, and occupies about half the volume of the float. 

 A prominent tubular stigma (figs. 4, 5, po) opens in the centre of its upper face. The 

 eight radial air-chambers, which surround the former as a regular corona (pq), are 

 crescentric, concave on the axial side, convex on the abaxial side ; each opens by a stigma 

 on the upper face {pe). The pneumatocyst possesses only eight simple tracheae (pt), one 

 arising from the inferior face of each radial chamber ; the eight tracheae descend obliquely 

 and contorted downwards, enter into the centradenia (fig. 4, uc), and terminate in it ; but 

 ihej do not pass into the exodermal wall of the central siphon. 



Centradenia (fig. 4, uc). — The large central gland is a circular disc, the diameter of 

 which is about three to four times as great as its height, and equals that of the 

 pneumatocyst. The inferior face of the latter is in close contact with the superior face 

 of the former ; whilst the inferior face of the centradenia is separated by a thick support, 

 the gastrobasal plate, from the base of the central siphon. The whole mass of the 

 solid centradenia is composed of exoderm cells. Its entodermal system of hepatic vessels 

 is confined to an eight-rayed star on its superior face. The eight radial canals, which are 

 connected in the centre of the latter, arise from the proximal third of the eight sub- 

 umbrellar radial canals which run from the base of the central siphon towards the margin 

 of the umbrella. This simplest form of the gastro-hepatic canal-system gives the explana- 

 tion of its more complex form in the other Disconectaa. 



Central Siphon (figs. 1-4, sa). — The central polypite is a thick-walled contractile tube, 

 inversely conical in the proximal half, cylindrical in the distal half. Its length about 

 equals the equatorial diameter of the umbrella, and is twice as great as the diameter of 

 its base. The thick muscular wall is composed of a strong outer layer of exodermal 

 longitudinal fibres, and a thin inner layer of entodermal circular fibres, both separated by 

 a structureless elastic fulcrum. The distal mouth is octolobate. 



The gastrobasis, or the horizontal roof of the stomach, is regularly octagonal, separated 

 by a thick fulcrum, the gastrobasal plate, from the centradenia above it. It is pierced in 

 the periphery by the gastral openings of the eight radial main vessels. These give off 

 branches for the gonostyles and tentacles, and run centrifugally in the subumbrella 

 towards the limbus, where they are united by a marginal ring-canal. 



Gonostyles (figs. 1-4, gs). — The eight sexual palpons are about half as long and broad 

 as the central siphon. They bear clusters of gonophores in their cylindrical basal part, 

 patches of cnidocysts in their spindle-shaped distal part. 



