Homologies of the Cephalopoda. 305 



alone agreeing with Cephalopoda in having a pedal flexure, that 

 of all the rest, except Nudibranchs and Tectibranchs (which 

 have scarcely any flexure at all), being cerebral. A point 

 of difficulty still remains respecting this. Although the heart 

 always accompanies the intestine in its flexures, and its affe- 

 rent vessels come from the direction of the anus, yet when the 

 intestine is rectified it appears to have a different relation to 

 it. When the intestine is naturally straight, as in the Nudi- 

 branchs &c, the heart lies on the cephalic side — I believe, 

 without exception. It would have the same position in the 

 Cephalopoda and Pteropoda. But in the Heteropoda (Atlanta) 

 certainly, in the Pulmonata, and, I think, also in the Pectini- 

 branchs it would lie on the pedal side. It is not easy to say 

 whether the heart lies within or without the curve formed by 

 the intestine (on which its position, when the latter is rectified, 

 would depend) when, in point of fact, it lies at the side. Per- 

 haps, however, it is all a matter of accelerated growth. In 

 the Lamellibranchs the intestine pierces the heart, which 

 therefore lies on both sides of it ; and the branchiae surround 

 the anus in Doris &c. Both tend to develop most on the 

 outside, while the portion lying Avithin the body aborts. Ac- 

 cording, therefore, as the flexure is cerebral or pedal does one 

 or the other part of the circle become persistent, under the 

 condition of being exterior. But the original flexure itself 

 may have been caused by the increased growth of that side 

 which now lies outside. This does not, certainly, account for 

 the one-sided hearts of the Nudibranchs ; but it does account 

 for the different direction in which the shell of a cephalopod 

 and of a snail is coiled : the former has its convexity, and 

 therefore its greatest growth, on the pedal, the latter on the 

 cerebral side. In both cases the convex side of the shell is on 

 the side of the heart. In the case of the Spirilla, not only the 

 convex side of the shell, but the whole last chamber of the 

 shell lies on the side of the heart. 



We should conclude from the above observations that the 

 Cephalopoda branched off from the main molluscan stem, 

 through the Pteropods, at an earlier period than the develop- 

 ment of ordinary Gastropods ; and, indeed, we find their re- 

 mains in deposits of earlier date than those containing the 

 latter. 



The next point of importance is the homology of the foot 

 and other non-pallial outgrowths, on which, in fact, depends 

 the position in which we should suppose the animal placed 

 for comparison. In Prof. Huxley's paper on the Morphology 

 of the Ccphalous Mollusca (Phil. Trans. 1853), the line 

 along the base of the foot is taken to have a constant direction, 



