REPORT ON THE SIPHONOPHORiE. 59 



Subumbrella (figs. 1-3, iv). — The inferior or basal face of the umbrella exhibits 

 beyond the deep submarginal ring-furrow a broad naked girdle, in which the radial canals 

 ascend from the tentacular zone to the margin of the umbrella. The tentacular zone 

 begins in the equator of the biconvex lens, and occupies the peripheral half of its lower 

 convexity. Its breadth equals the radius of the central disc of the exumbrella. 



Pneumatocyst (fig. 4, from above ; fig. 5, from below; fig. 3,p, in vertical meridional 

 section). — The float, filled with air, is campanulate, and envelops the subspherical centra- 

 denia, with exception of its basal centre, which is occupied by the central siphon. The 

 equatorial diameter of the pneumatophorous bell is twice as great as its vertical main 

 axis. The basal opening of the bell, closed by the base of the central siphon, is scarcely 

 greater than half its height. The concave inferior face (or the cavity of the bell) is in 

 close contact with the centradenia, and the thirty-two prominent radial crests of the 

 former fit into thirty-two deep radial furrows of the latter. The convex superior face 

 is flatly conical above the equator (itg) ; its peripheral girdle, beyond the equator, is 

 regularly divided into sixteen furcate radial lobes (or thirty-two smaller lobes). 



The central chamber of the pneumatocyst is surrounded by a regular girdle of eight 

 radial chambers, each of which possesses a stigma on its upper, and a trachea on its 

 lower side. This octoradial girdle is surrounded by eight to ten complete, concentric, 

 circular ring-chambers, the outermost of which touches the equator of the lens. Outside 

 follows the peripheral girdle of thirty-two lobes, which is recurved inwards and down- 

 wards (horizontally expanded by pressure, see fig. 4). The superior or convex face of 

 the pneumatocyst bears in its superior central part (which is in contact with the exum- 

 brella) numerous radial rows of stigmata. From its inferior or concave face, which is in 

 contact with the centradenia, hang down thirty-two prominent radial crests, or rather 

 lamellar pouches (fig. 6) ; and from the lowermost part of the latter arise numerous long 

 tracheae which enter into the glandular tissue of the liver (fig. 3, uc) ; the majority of 

 them terminate here with open distal ends, whilst a small part of the tracheae enters into 

 the basal part of the wall of the siphons, and ends in its exodermal epithelium. 



The chitinous substance of the thin wall of the campanulate pneumatocyst is very 

 delicate and richly folded, like crumpled tissue-paper. 



Centradenia (fig. 3, uc). — The large central gland is nearly spherical, and entirely 

 fills up the subumbrellar cavity of the pneumatocyst. The circular polar area only of its 

 lower pole is in contact with the base of the central siphon. Besides this smooth inferior 

 area, the entire surface of the centradenia is traversed by numerous deep meridional 

 grooves, which are filled by the lamellar radial pouches of the basal face of the pneuma- 

 tocyst. The dense parenchyma of its exodermal cellular tissue is traversed by numerous 

 bent tracheae, and by a loose network of hepatic canals. The latter arise from a regular 

 octoradial star of superior liver-canals, which unite in the superior or apical pole of the 

 centradenia. These eight radial main canals embrace the greater part of its surface like 



