50 THE VOYAGE OF H.M.S. CHALLENGER. 



between fifty and ninety short trachea) (six to twelve in each octant). These are placed 

 in the periphery of the octoradial ring, and in the two to four next following chambers ; 

 they are wanting in the periphery. 



Centradenia (figs. 10-12, uc). — The large central gland, or so-called "liver," is an 

 octagonal, lenticular, biconvex disc, of the same horizontal diameter as the overlaying 

 pneumatocyst. Its thickness is inconsiderable, and decreases towards the periphery. 

 Its convex superior face (fig. 10) is in close contact with the concave inferior face of the 

 latter. The peripheral octagonal margin of the centradenia does not exceed that of the 

 covering pneumatocyst. 



The inferior face of the centradenia is rather flat, and is separated by a thin fulcrum 

 from the subumbrella. Its central part is in contact with the gastrobasal plate of the 

 central siphon ; its peripheral part with the corona of gonostyles, and its margin with the 

 corona of tentacles. 



The structure of the centradenia exhibits in Disconalia the same remarkable simplicity 

 and typical form as in Discalia. The solid parenchyma is entirely composed of masses 

 of aggregated exoderm-cells, with innumerable cnidocysts. A regular octoradial star 

 of pigmented hepatic vessels is placed on its superior face (fig. 10). From the centre of 

 the latter arise eight perradial brown " fiver-canals," which branch dichotomously, and 

 run on its upper face towards its peripheral margin. They pass here over into the 

 peripheral canal-network of the subumbrella, from which arise the canals of the tentacles 

 and gonostyles. 



Central Siphon (PI. L. figs. 1, 3). — The large central polypite has the usual form 

 of an inverted cone. The broad octagonal base occupies the central area of the sub- 

 umbrella, and is separated from the centradenia above it by the gastrobasal plate. The 

 periphery of the base opens by eight perradial ostia (PL XLIX. fig. 12) into the eight 

 radial main vessels which run in the subumbrella towards the limb, and give off the 

 ascending centripetal canals of the " liver-star " (fig. 10). 



The muscular wall of the central siphon is very thick. It opens by a distal octolobate 

 mouth, the edges of which are strongly armed with cnidocysts (PL L. fig. 3). 



Gonostyles (PL L. figs. 1, 2). — Sixteen sexual palpons, or mouthless gonostyles, form a 

 corona around the base of the sterile central siphon. They are small spindle-shaped sacs, 

 scarcely half as long as the central siphon. Their wider basal half bears clusters of 

 gonophores (fig. 2, g), whilst the slender distal half is beset with cnidonodes (k) ; the 

 closed distal end is obtusely conical and entirely covered by cnidocysts. 



Tentacles (PL L. figs. 1, 4-G). — The tentacles are very numerous and densely 

 crowded in the subumbral groove between the corona of gonostyles and the limb of the 

 umbrella. They alternate here in a double row (PL XLIX. figs. 1, 2). The tentacles 

 which arise from the eight perradial corners of the octagonal girdle are much longer than 

 the interjacent interradial, and form therefore eight prominent bunches. 



