Homologies of the Cephalopoda. 309 



turns to the cerebral side, we have the " velum " formed on 

 that side, whereas in the Cephalopoda, the flexure being to the 

 opposite side, we have what we may call the " an ti velum " 

 on the pedal side. Thus the arms are homologous to the oppo- 

 site portion of the architroch to that which forms a velum, and 

 merely afford another instance in which these primitive for- 

 mations are retained as functional organs. Moreover, from 

 within the circle of the embryonic velum rise up in some 

 Gastropods two long retractile tentacles ; in like manner 

 from within the later-formed circle of the antivelum rise up 

 the two retractile tentacles of the Decapods. 



If such be the true homology of the arms, what in the 

 Cephalopoda represents the foot of other Mollusca? When 

 Ave remember that even among the Lamellibranchs the foot is 

 sometimes wanting, that it is very variously developed in the 

 Gastropoda, and has merely a rudimentary representative in 

 most of the Pteropoda, we cannot make sure of its being- 

 present at all. That it should be represented by the two 

 halves of the funnel, as Gegenbaur supposes, is as objection- 

 able an idea as its homology with the arms, and for the same 

 reason — though, being more closely connected with the region 

 of the foot, they may be supplied from the pedal ganglia. I 

 can only suggest one median unpaired outgrowth which may 

 represent it ; and that is the valve within the funnel, which 

 occurs in a great number, and especially in the Nautilus, 

 which is least removed from the general type. This, however, 

 must be doubtful, as the development of this valve has not 

 been observed. 



The recognition of the two funnel-halves of the adult 

 Niutilus and the embryonic Dibranchiate as part of a second 

 outgrowth surrounding the body, to which the name of epi- 

 podium has been given by Huxley, is pretty general ; and 

 there seems to be nothing against it. Grenadier has shown 

 that each half is originally again in two parts, one following 

 the other longitudinally; and one of these parts only he 

 reckons homologous to the sails of the Pteropods. 



With regard to the relations of the Nautilus to other Cepha- 

 lopoda, it is remarkable how every additional fact in thedevelop- 

 ment of the latter shows the former to represent embryonic 

 stages; and this is the more interesting as the allies of the Nau- 

 tilus certainly preceded the Dibranchiates in their appearance 

 on the globe. The following points, old and new, are most 

 noticeable in this respect. In the Nautilus and in the embryo 

 Dibranchiate the funnel is in two halves, but unites into a 

 single tube in the adult of the latter. In the Nautilus and 

 embryo Dibranchiate up to a late stage there is no ink-bag, 



