1894 - 95 .] Dr Gilchrist on Torsion of the Molluscan Body, 369 
cause increases the probability against the latter solution being 
applicable to all cases. 
Finally, we may note the significance of the indications we have 
met with in the Tectibranchiata, that there are two lines of develop- 
ment, according to the relative preponderance of the mantle or foot. 
We have traced these through intermediate forms to what appear 
to be two corresponding groups in the FTudibranchiata. Judging 
from this evidence alone, therefore, we would presume that the 
cerata are neither the homologues of the mantle (Pelseneer) nor of 
the foot (Herdman), but that they may be the one or the other in 
different cases, and there is no reason why they should not some- 
times be both. Professor Herdman has shown that the cerata are 
not innervated exclusively by the pleuro-visceral nerves (mantle 
nerves), as was previously supposed, nor by the pedal nerves 
(pleuropodial nerves), but that they are innervated from the one 
or the other, or both sources, in different forms. He infers from 
this that here innervation is not to be trusted as a guide to homo- 
logy of parts. If, however, these apparently conflicting facts be 
taken in conjunction with the lines of development in the imme- 
diate ancestors of the group, it will be apparent that they afford 
valuable confirmatory evidence for the conclusions here arrived at, 
viz., that the various forms in the Nudibranchiata are to be referred 
back for interpretation to the Tectibranchiata, where there may be 
preponderance of the mantle on the one hand or the foot on the 
other, or maybe equal development of both, for it is not to be 
supposed that the two lines of development are the only alterna- 
tives, or even that they are exclusive of each other (Doris ?). Other 
variations certainly exist in the Nudibranchiata, only a few forms 
of which we have considered. 
[Explanation 
