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



cysts, and (usuall) T ) exhibits a distinct octoradial pigment star in its centre (fig. 7). The 

 internal or inferior lamella is the pneumatosaccus, the invaginated plate of the exoderm, 

 which secretes the chitinous pneumatocyst. Its lower face is in contact with the npper 

 face of the centradenia. 



Limbus Umbrella} (PI. L. figs. 1, 9, us). — The free horizontally prominent margin of 

 the umbrella, which separates the exumbrella from the subumbrella, is in all Discalida? more 

 or less octolobate ; the eight interradial convex lobes are the more prominent, the deeper 

 are the eight incisions in which are inserted the marginal tentacles. The whole edse of 

 the mantle-border is beset with a continuous series of marginal muciparous glands. 



Subumbrella. — The inferior (basal or distal) face of the umbrella is convex, and bears 

 in its centre the large conical central siphon, around this a corona of gonostyles (eight 

 in Discalia, sixteen in Disconalia), and in the peripheral part a simple or double corona 

 of tentacles (eight simple tentacles in Discalia, PI. XLIX., eight radial bunches of 

 numerous tentacles in Disconalia, PI. L.). 



Pneumatocyst (PI. XLIX. figs. 4, 5, 8, 9). — The float filled with gas, which is in- 

 cluded in the pneumatosaccus, always exhibits in the Discalidse a regular octoradial 

 structure. This is of typical simplicity in Discalia (PI. XLIX. figs. 2-5), composed only 

 of a subspherical central chamber and a surrounding regular ring of eight equal triangular 

 radial chambers. The more advanced genus, Disconalia (figs. 8, 9), exhibits the same 

 biconvex octoradial disc in its central part ; but it is here surrounded by a peripheral 

 girdle of five to ten concentric ring-chambers ; the middle ones of these are far broader 

 than the innermost and the outermost. 



Pncumothyrse. — Each of the eight radial chambers of the central disc of the pneuma- 

 tocyst communicates with the common central chamber by an inner opening or pneumo- 

 thyra, placed on the proximal apex of the triangular chamber. Opposite to this lies in 

 the centre of its distal base another pneumothyra, which opens into the first or innermost 

 ring-chamber. An interradial series of similar septal openings, by which every two 

 neighbouring chambers communicate, lies in the centrifugal continuation of the interradial 

 line, which bisects each triangular chamber and connects its apical with its basal pneumo- 

 thyra. They are, therefore, in Disconalia eight regular interradial rows of pneumothyi'ae 

 (PI. XLIX. figs. 8, 9, pg), and these alternate regularly with the eight perradial grooves 

 which separate the eight triangular chambers from one another, are continued to the j»eri- 

 pheral margin of the pneumatocyst, and divide the latter into eight equal triangular 

 octants. The free margin of the pneumatocyst thus becomes distinctly octolobate. 



Stigmata (PL XLIX. figs. 2, 5, 8, 9). — The superior (apical or proximal) face of the 

 pneumatocyst bears the stigmata or the short tubular openings which pierce the ex- 

 umbrella and permit an expulsion of the enclosed air. Discalia (fig. 2) has only nine stig- 

 mata, one central, in the central chamber, and one in each of the surrounding eight radial 

 chambers. In Disconalia (figs. 8, 9) this number is increased by a variable number of 



