62 THE VOYAGE OF H.M.S. CHALLENGES. 



campanulate double-walled float is filled up by the ceutradenia (uc), which passes through 

 its basal aperture (fig. 4). 



An exquisite regular octoradial structure is visible in all parts of the pneumatocyst. 

 The central chamber (ph) is nearly cylindrical, with an apical stigma (po), and sur- 

 rounded by a regular corona of eight radial chambers (p>q), each with a stigma above 

 and a trachea below. The deep radial furrows between these eight lobes (fig. 7, pr) are 

 prolonged to the periphery of the pneumatocyst, which is divided into eight large main 

 lobes. Each of the latter is again divided by a median incision into two secondary lobes, 

 and these again into four tertiary lobules. Thus the entire peripheral part of the float, 

 beyond its equator, is radially lobate. The deep radial furrows of the superior face of 

 the pneumatocyst correspond to high prominent radial crests or lamellar pouches of 

 its concave lower face, and from the height of these lamellar crests arise numerous 

 tracheae entering into the centradenia. The double wall of the concentric ring-chambers 

 presents therefore a very large surface, by means of the strong development of these 

 radial folds. 



Centradenia (figs. 6 and 9, transverse sections ; fig. 4, uc, meridional section). — The 

 large central gland exhibits in this Porpitid a strange cylindrical form, due to the 

 unusual development of the vertical or longitudinal axis ; the latter attains 3 to 4 mm., 

 whilst the horizontal diameter of the cylindrical centradenia is only 1 to 1*5 mm. Its 

 middle part is constricted by the embracing inferior margin of the campanulate pneu- 

 matocyst. The radial lamellar crests of the latter fit into corresponding meridional 

 furrows on the surface of the upper head-like part of the centradenia. The lower 

 cylindrical part of this latter is surrounded by the corona of sexual siphons, and its 

 circular basal face is in contact with the basal plate of the sterile central siphon. From 

 the periphery of this plate arise eight radial canals, which are soon forked ; an outer 

 main branch runs outwards to the tentacular zone and the umbrella margin ; an inner 

 main branch ascends vertically between the convex outer surface of the centradenia and 

 the concave inner surface of the pneumatocyst. These eight centripetal liver-canals 

 unite in the apical pole of the central gland, and form here a regular "liver-star" (fig. 8, 

 cm) they give off numerous branches, which form a network in theexodermal parenchyma 

 of the liver ; between its branches numerous bent tracheae are visible. 



Central Siphon (sh, figs. 2 and 3 in profile, fig. 4 in vertical section, fig. 5 from below, 

 fig. 10 in transverse section). — The large central polypite has an upper pear-shaped part 

 (stomach) and a lower cylindrical part (proboscis). Its length is, in the contracted state, 

 equal to the diameter of the spherical umbrella, and twice as great as its greatest hori- 

 zontal diameter (near the base). Its thick muscular wall exhibits eight longitudinal 

 exodermal furrows on the outside, and eight corresponding radial entodermal folds on the 

 inside (fig. 10). Between these folds open in the basal part the eight radial canals 

 (fig. 9, sf). The distal mouth has eight radial lappets (figs. 3, 4, so). 



