554. Ww. A. HERDMAN AND J. A. CLUBB. 
homology, we arrive at the remarkable result that these 
processes of the body-wall are not all of the same nature; and 
that whereas in Tergipes, and possibly also in Facelina, 
they may be considered as pedal in origin, and as homologous 
with the epipodia of an ordinary rhipidoglossate Gastropod, 
such as Trochus (where the epipodial ridges and processes are 
supplied, according to Pelseneer, by nerves arising from the 
dorsal part of the pedal ganglia), in Polycera, Ancula, and 
others, they must be regarded as totally distinct structures of 
pallial origin. This seems to be a clear case of reductio ad 
absurdum. 
We have in a former paper tried to show that these pro- 
cesses, whether ridges or papille or lobes, parieto-cerata as in 
Dendronotus and Tritonia, or hepato-cerata as in Eolis 
and Doto, are all really modifications of the same thing; and 
although it might conceivably be argued that the parieto-cerata 
(pleural) of Ancula might be different in their nature and 
origin from the hepato-cerata of Kolis (mainly pedal), still no 
one would be likely to suggest that the cerata of Tergipes 
(pedal) and of Hermza (pleural) are not homologous struc- 
tures. And in addition there are the intermediate conditions 
found in Dendronotus and in Marionia,’ linking together 
the purely pedal and the purely pleural methods of inner- 
vation. 
Consequently we are inclined to consider that in this case 
the nerve-supply cannot be taken as a sure indication of the 
homology, and that possibly the innervation has undergone 
modification in accordance with changes in the position, size, 
and relation to other organs of these ceratal processes in the 
Nudibranchiata. The cerata, which we still regard as homo- 
logous structures throughout the series of Nudibranchiata, 
must, from their great differences in size, shape, colours, sting- 
ing properties, and contents, be of very varied importance to 
1 T. e. from the integument dorsal to the foot, and supplied by the pleural 
ganglia, whether there is a distinct “ pallium” present or not. 
2 Where, according to Vayssiére, the main epipodial nerves are pleural; 
but there are also smaller accessory nerves from the pedal ganglia. 
