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



The mouth or the distal opening of the central siphon is either circular or octagonal, 

 with eight short regular lips or radial mouth lobes (PI. L. figs. 1, 3). Its edge is 

 strongly armed with cnidocysts. 



Centradenia (PI. XLIX. figs. 4; 10, 11). — The large central gland, which is called 

 the " liver " in the Porpitidse and Velellidse, does not exhibit in the Discalidse the coin- 

 plicated structure seen in those two families, but merely a typical and most instructive 

 simple shape. It is a biconvex lenticular disc of circular or octagonal outline, in Discalia 

 (fig. 4) relatively thick and small, in Disconalia (fig. 10) broader and flatter. Its 

 horizontal diameter is two to four times as great as its vertical main axis, and of the 

 same length as that of the pneumatocyst above it. Its peripheral margin is surrounded 

 by the corona of gonostyles, whilst its inferior face is in contact with the central siphon. 



The entire mass of the lenticular centradenia is composed, in the Discalidae, of 

 numerous densely aggregated exodermal cells and cnidocysts, and many of these are (in 

 the well-preserved spirit specimens of the Challenger collection) filled with an air-bubble ; 

 it is therefore very probable that these cells secrete the gas, which is taken up by the 

 open distal ends of the tracheae, and conducted by these into the chambers of the 

 pneumatocyst. These gas-producing exodermal cells are probably derived from the basal 

 part of the pneumatosaccus, or the invaginated lamella of the exoderm which includes 

 the pneumatocyst. The thin structureless supporting plate, which separates the upper 

 face of the centradenia from the overlying pneumatosaccus, is pierced by numerous pores 

 which permit a direct connection between the two. 



The solid exodermal parenchyma, in the Discalidee, is only traversed by the trachese, 

 and not by the so-called " liver-canals," which form a complex network in the Porpitidse 

 and Velellidse. These hepatic canals are here confined to an octoradial " liver-star," 

 which lies in the superior face of the centradenia. The eight main rays of it lie in the 

 eight perradial grooves between the eight interradial triangular air-chambers, and are 

 united in the centre of the lower face of the central chamber. They arise from the basal 

 part of the eight subumbrellar radial canals (near their opening into the base of the central 

 siphon), and embrace the surface of the centradenia like eight equidistant meridional 

 arches. They remain single in Discalia (fig. 4), whilst they are forked and branched 

 dichotomously in Disconalia (fig. 10). 



Gonostyles. — The polypites (or secondary manubria) which produce by budding the 

 medusiform gonophores are in the Discalidse mouthless palpons, and not siphons pro- 

 vided with a mouth, as is the case in the nearly allied Porpitidaa and the more divergent 

 Velellidse ; but also in the latter two families the gonostyles arise from the subumbrella 

 in the same mouthless form, and accpure their mouth opening later. Their structure is 

 the same as in the palpons of the Discalidaa. These are spindle-shaped or pyriform, 

 much smaller than the central siphon ; they form a regular simple corona around the 

 base of the latter. Discalia (PI. XLIX. figs. 1, 3) possesses eight, and Disconalia 



