366 Proceedings of Eoyal Society of Edinhurgh. [sess. 
nephridial openings are practically in the middle line, and are 
surrounded by the gill ; but so different is the structure of this gill 
that it might well be a new formation. Again, we may ask are the 
dorsal processes or cerata (cer.) the homologues of the pleuropodia ? 
The genital opening is now near the head region, in the position of 
the anterior end of the genital furrow (fig. 5, gen. /.) in previous 
forms. 
In the case of Fiona (fig. 7) the gill has disappeared, and the 
function of respiration seems to be entirely performed by the dorsal 
processes, which have greatly increased in number. 
Fig. 7. 
In both these forms we meet, for the first time, with a total dis- 
appearance of the osphradium along with the pallial cavity. There 
are, however, well developed rhinophora in the head region. 
As to the homology of parts here suggested, it cannot be main- 
tained with absolute certainty ; but the line of development we 
have been following in the Tectibranchs has prepared us for every 
one of the transformations we now meet with in these Nudibranchs. 
Another line of development is, however, to be traced from the 
Acera type. The mantle may preponderate in development, causing 
a considerable displacement of the pallial organs, not as before, 
towards the dorsal surface, but rather laterally and ventrally. The 
