REPORT ON THE SIPHON OPHOR^E. 65 



clusters of gonopkores) ; the second zone is marked by the corona of tentacles, divided 

 into sixteen bunches ; and the third, outermost, zone is formed by the inferior face of 

 the free peripheral limb. 



Pneumatocyst (fig. 4, superior or apical view; fig. 5, inferior or basal view; figs. 3, 6, 

 7, p, meridional sections). — The float, filled with gas, is a circular disc, the horizontal 

 diameter of which (9 to 12 mm.) is ten to twelve times as great as its vertical diameter 

 (1 to 1*2 mm.). Its thickness is nearly equal throughout, or increases a little towards 

 the margin. The superior or exumbrellar face (fig. 4) is slightly convex, and exhibits 

 numerous denticulate radial ribs, sixteen of which are more prominent (fig. 1). Numerous 

 stigmata (pe) open by the conical papilla? of the radial ribs. The inferior or subumbrellar 

 face (fig. 5) is slightly concave and radially folded, numerous radial furrows (sixteen of 

 which are deeper) corresponding to the ribs of the upper face. Numerous articulate 

 trachea? (figs. 5, 6, pt ; fig. 8) arise from conical protuberances of the inferior ribs, which 

 are prominent between every two radial sulci.' 



The central chamber of the pneumatocyst (figs. 3, 6, ph) opens above by a central apical 

 stigma (po), and is surrounded by a regular corona of eight triangular radial chambers ; 

 each of these bears a circular stigma (fig. 4, pe) on its upper face (placed in variable 

 points), whilst from its lower face arise one or two short tracheae. The corona of con- 

 centric ring-chambers, which surround the girdle of eight radial chambers, is composed of 

 nine to twelve rings. The height (or vertical diameter) of these rings increases from the 

 centre towards the periphery, whilst the breadth (or horizontal diameter) decreases (figs. 

 3, 7, pk). The inner or proximal wall of each ring is concave, the outer or distal wall 

 convex. The concentric rings communicate one with another by eight radial rows of 

 apertures (figs. 3, 4, pg), which are placed interradially between the perradial sulci. The 

 innermost or first ring embraces the octoradial chamber-girdle not only from the distal, 

 but also completely from the basal side (figs. 3, 6, pk) ; the other concentric rings 

 embrace one another only from the distal side. 



Centraclenia (figs. 3, 6, 7, uc). — The large central gland (or the so-called liver) is a 

 circular biconvex lens, the horizontal diameter of which (6 to 8 mm.) is three to four 

 times as great as the vertical axis (2 mm.). Its upper surface is in close contact with 

 the concave lower surface of the pneumatophore, its lower surface with the subumbrella. 

 Numerous radial ribs of the upper surface fill up the radial sulci of the lower surface of 

 the pneumatophore. The brown liver vessels are very densely crowded in the upper 

 half, loosely scattered in the lower half of the pneumatophore. All these hepatic canals 

 unite on the lower surface of the centradenia into descending canals, which pierce the 

 fulcrum of the subumbrella ; eight of these (forming an inner girdle of openings) open 

 into the peripheral part of the base of the central siphon ; sixteen others (forming an 

 outer girdle) open into the sexual siphons (sx). The majority of the canals of the central 

 gland contains in their epithelium that black pigment which is regarded as " hepatic 



(ZOOL. CHALL. EXP.— PAKT LXXVII. — 1888.) Hhhh 9 



