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



less distinct, of the same octoradial composition of the centre of the float, which is 

 immediately evident in all Porpitidse and Discalidse. 



The pneumatocyst of Rataria consists only of a horizontal, slightly campanulate or 

 flatly conical disc of elliptical outline, whilst that of Velella and of Armenista bears a 

 solid vertical triangular crest, placed in a diagonal axis of the disc. 



The pneumatocyst of Rataria is of the highest interest, since it offers all desirable 

 intermediate forms between the bilateral crested float of the other Velellidaj and the 

 circular crestless float of the Porpitidse. This is the more important, as the young 

 Ratarula-larvse of Velella and Armenista exhibit the same structure of the pneumatocyst 

 (during a longer or shorter time) which is permanent in the autonomous genus Rataria. 

 A central chamber, placed in the centre of the exumbrella, is here always surrounded by 

 eight radial chambers, just as in the Porpitidae and Discalidse. But whilst in these two 

 families each radial chamber (as well as the central chamber) possesses a stigma, or a free 

 opening on its upper surface, there are in the similar pneumatocyst of Rataria three 

 such openings only, one in the upper face of the central chamber and one on each side of 

 it, in the two chambers, which are directed towards the two sagittal poles, or the poles of 

 the major axis of the ellipse. The stigmata of the six other chambers seem to be lost by 

 phylogenetic reduction (PI. XLIV. fig. 8). Each of the eight radial chambers usually 

 possesses a trachea or a bunch of descending air-tubules on its lower face (fig. 9). 



The octoradial structure of the pneumatocyst, which is obvious in Rataria, is also 

 recognisable in the similar Ratarula-larvse of Velella and Armenista. But it is not 

 ecmally distinct and well preserved in all species of these two genera. The peripheral 

 wall of the central chamber, which separates it from the eight surrounding radial 

 chambers, is pierced by eight openings, connecting the former with the latter. These 

 openings of communication afterwards often become so enlarged that the eight chambers 

 appear only as radial lobes of the central chamber, and sometimes the eight radial septa 

 between the former are so reduced that the eight lobes nearly disappear. This fact 

 explains the striking contradictions of the former observers, some of whom interpret the 

 Ratarula-larvse as young forms of Velellidse (Huxley, Bedot, &c), some as larvae of 

 Porpitidse (Agassiz, Burmeister, &C.). 1 Indeed all these larvse belong to the Velellidse, 

 since the larvae of the Porpitidse never develop a vertical sail. But the octoradial struc- 

 ture of the central part of the pneumatocyst, which is always very distinct in the latter, 

 is of very variable distinctness in the former ; sometimes it is well preserved, at other 

 times not, and sometimes the cenogenetic modification is so strong that it becomes quite 

 lost. 



The concentric ring-chambers of the pneumatocyst, which surround its octoradial 

 central part, are not circular in the Velellidse (as in the Porpitidse and Discalidse), but 

 more or less elliptical, with prolonged sagittal axis, and shortened transverse or frontal 



1 Compare Pagensteclier, 55. 



