144 DISCOVERY REPORTS 



the left 1 ridge in the Ceratocymba group, runs down to the mid-dorsal tooth, and not to the left tooth. 

 In species of Abyla (sensu stricto), but not in those of Ceratocymba, the ventro-lateral facet of the 

 anterior nectophore is divided by a horizontal ridge. 



As far as I can follow Moser (1925, p. 314) she said that the condition of the teeth and ridges at 

 the distal end of the posterior nectophores of C. sagittata and C. lenckartii is the most primitive and 

 most like Diphyes, and I agree with her. In Ceratocymba dentata she said that the median dorsal 

 (Moser's ventral) ridge has shortened and is displaced to the right, together with the middle (dorsal) 

 tooth. In C. dentata I have not seen such a displacement to the right, but I have seen a slight displace- 

 ment towards the left. In C. leuckartii, however, the middle tooth is much closer to the left lateral 

 than to the right lateral ridge. Moser appeared to believe that in the trigona-group the dorsal (Moser's 

 ventral) ridge and dorsal tooth had disappeared altogether, that the tooth at the oral end of the left 

 lateral ridge is the left lateral tooth, and that the tooth furthest to the left is a new structure, the 

 'pseudo-tooth'. 



In reality I think that the middle tooth and the distal end of the left lateral ridge have come together 

 so that the left lateral tooth has no corresponding ridge (PI. IX, fig. 4). The beginning of the process 

 can be seen in C. leuckartii, where the left ridge and middle tooth are close together. In fact, in 

 C. leuckartii Moser's joint left and right ('ventral ') dorsal facets are presentjustastheyarein C. dentata. 

 But the middle (median dorsal) tooth is at the left margin of this joint dorsal facet in the trigona-group 

 instead of in its middle line as in C. dentata. One might almost say that in the trigona-group the 

 reduced median dorsal ridge and tooth have coalesced with the left lateral ridge. Moser thought that 

 the dorsal (her ventral) tooth had completely disappeared, together with its ridge, and that a ' pseudo- 

 tooth' had been developed on the left side to compensate for this. But there is no need to invent 

 a 'pseudo-tooth', for of course in all abylines there are three dorsal teeth at the distal end of the 

 posterior nectophore, and it seems to be so much more probable that they are homologous structures 

 in all species. 



One of the most striking features of the nectophores of abylines is the peculiar angle between the 

 dorso-ventral planes of the anterior and posterior nectophores. Viewed from aft (PI. IX, fig. 4), the 

 distal end of the posterior nectophore in some species is twisted clockwise through an angle of from 

 55 to 90 so that the dorsal sides of the two nectophores are no longer opposite one another. The 

 twisting is accompanied by a progressive broadening of the left wing of the hydroecium in species of 

 the trigona-group. Here, too, the distal edge of the broad left wing is marked by a double row of 

 spines, an inner and an outer row as can be seen from a distal and ventral view. These two rows of 

 spines are much better marked in Ceratocymba dentata (PI. X, figs. 4, 5). 



The starting-point of this evolutionary trend of axis-twisting and wing-broadening can best be seen 

 in the homologous structures of the posterior nectophore of Diphyes dispar, where the distal edges of 

 both right and left wings of the hydroecium are formed into a flattened area without any trace of 

 spines. In that species the ventral edges of both hydroecial wings reach the level of the velum and 

 then bifurcate, turning distad to form the flattened area just described. 



No specimens of those species of the trigona-carina type occurring in the Atlantic have been found 

 in the 'Discovery' material from the Indian Ocean, though Dr Sears (1953) has recorded the capture 

 of A. trigona from that ocean at 'Dana' Stations 3920, 3921 and 3955. The specimens consisted of one 



1 My terminology differs from that of Bigelow & Sears (1937, p. 4) because I use the same orientation (oral and aboral) 

 for an adult as for a larva, where it is self-explanatory. Unfortunately these animals progress with the aboral end forwards. 

 By a very old convention the ventral side of a larva is the one on which budding first appears. 'Ventral' has thus become 

 to be synonymous with adaxial, and 'dorsal' with abaxial. As far, then, as orientation of posterior nectophores of diphyids 

 is concerned my 'left' and Moser's 'left' corresponds with Bigelow's 'right' and Sears's 'right'. 



