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



umbrella of the Velellidse may be called either amphitheet or bilaterally-radial. It is 

 characterised by three unequal dimensive axes, each perpendicular to the other two ; one 

 being allopolar (with unequal poles), the other two isopolar (with equal poles). The allo- 

 polar axis is the vertical main axis, its upper or apical pole determined by the top of the 

 sail, its lower or basal pole by the mouth of the central siphon. The greater isopolar axis 

 is the sagittal axis of the disc (the major axis of the ellipse), which divides it into two equal 

 symmetrical halves, right and left. The smaller isopolar axis is the transverse or frontal, 

 which is perpendicular to the latter, and separates the disc also into two equal halves, 

 dorsal and ventral ; it corresponds to the minor axis of the ellipse. 



The octoradial character of their amphitheet ground-form is always indicated by the 

 origin and course of the eight primary gastro-canals, which arise from the base of the 

 central siphon ; secondly, it is marked by the centre of the pneumatophore, which 

 exhibits more or less distinctly eight radial chambers around the simple central chamber ; 

 thirdly, it is often indicated by the radial arrangement of eight primary marginal 

 tentacles and of other parts. The umbrella of the Velellidse is therefore composed originally 

 of eight equal parameres (or homotypical radial parts), like that of the Porpitidse and 

 Discalidse ; but it differs from that of the latter in the fact that the parameres are dis- 

 posed in pairs, so that the disc may be regarded also as composed of four paired quadrants ; 

 and sometimes the limits of these are sharply marked off by four marginal incisions of 

 the pneumatophorous disc. 



The quadriradial appearance, the body seeming to be composed of four parameres (as 

 in the most Medusge), is mainly obvious in the more highly developed Velellidse. But 

 regarding their phylogenetic origin from the Porpitidae, we must assume that this funda- 

 mental form has arisen from a true octoradial type composed of eight parameres. In no 

 case can we regard this fundamental form as uniradial, a point of view which is repre- 

 sented by Chun. 1 I agree perfectly with the explanation of the origin of the sail which 

 Chun there gives (loc. cit., p. 15); but I cannot agree with his promorphological 

 deductions. I cannot concede at all that uniradial fundamental forms exist anywhere. 



A true asymmetry, mentioned by many authors in the umbrella of the Velellidae, 

 does not exist ; for in every case the two halves of the body, which are separated by any 

 possible meridional section (through the vertical main axis), are perfectly equal ; and the 

 dorsal and ventral halves cannot be distinguished by any character ; the right and left 

 halves being likewise equal one to another. The so-called " asymmetry" in the disc of 

 many Velellidse is only the beginning of a spiral twisting, caused by a slight dislocation 

 of the vertical sail, turning more or less around the vertical main axis. But in the 

 elliptical Rataria (and also in the similar Ratarula-lkrvse of Vclella and Armenista) the 

 sail is placed in the sagittal plane, and its later dislocation from this plane is a secondary 

 alteration, comparable to the turning of the sail in a boat sailing before the wind. 



1 Chun, Fauna and Flora des Golfes von Neapel, i., Ctenophoren, 1880, p. 14. 



