514 APPENDIX. 



with the addition of a mass of the most minute anatomical 

 and functional details to us so incomprehensible, that we 

 have given up the attempt to unravel them. 



The key-stone of the theories of these gentlemen is the 

 assumed permeability of the acephalan gill-laminae. I believe 

 that this determination will prove erroneous. I am, of course, 

 open to conviction, but I think my position founded on the 

 rock of " magna est veritas, et prsevalebit." 



Under these circumstances I cannot, in the present phase 

 of this question, withdraw a single position of the branchial 

 theory I have proposed. 



Anatomy, even the microscope, chemical tests, and the 

 most ingenious experiments, have hitherto failed demon- 

 stratively to solve this long-vexed question ; all our attempts 

 amount to little more than hypothesis, intrepid assertion and 

 assumption, which often simulate facts, and pass for proofs. 



DENTALIUM TARENTINUM. (P. 235.) 



I have stated that "M. Deshayes has omitted to mark the 

 vein which runs at the dichotomous points of his organ, 

 which, when viewed under high powers, is very visible, and 

 which I take to be the branchial vein." 



I withdraw this observation, whether my conjecture be 

 correct or not, as it would have been inconsistent in that 

 naturalist to figure a branchial vein in an organ considered 

 by him to be the liver. 



His liver is our branchiae. We have lately re-examined the 

 Dentalium Tarentinum, and we almost think that the artery 

 and vein have a nearly parallel position on the dorsal line, as 

 in the Bivalves. The tumid cord-like appearance of the lower 

 border of this minute organ has perhaps led to an erroneous 

 conclusion. 



CHITON CANCELLATUS. (P. 251.) 

 CHITON ALBUS. (P. 251.) 



Though the above are admitted as distinct species, we are 

 now bound to add, that this determination must be considered 

 provisional ; there is reason to believe that both these forms 



