MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 57 
have detailed add ocular proof to his already convincing- demonstrations. But it is 
to be observed, that in this respect, again, the “ highest ” Cephalopod, Octopus, 
possesses no “ higher” organization than the Slug or Snail* * * § 
The consideration of the archetypal vascular system leads naturally to that of the 
value of the distinction made by M. Milne-Edwards between oplsthohraiicMate and 
prosohrancMate Mollusca. If my views be well-founded, it is clear that opistho- 
branchisni is the typical condition of all Mollusks, and that prosobranchism is one 
result of that asymmetrical development which I have endeavoured to show to be 
the principal agent in modifying the form of these animals. A little consideration 
will render it evident, that neither the neural nor the haemal flexure will have any 
effect in altering the position of the heart, so long as the flexure occurs hehind it, 
while eitlier flexure will produce prosobranchism, if it take place before the heart. 
Prosobranchism then indicates that a flexure has taken place, but not in what 
direction ; opisthobranchism indicates only that no flexure has taken place in front 
of the heart. 
As derived characters, therefore, we may expect them to fail in certain cases ; and 
those Mollusks which I have chosen to illustrate this paper are instances of their 
failure. Firoloides is nearly opisthobranchiate, while Atlanta is very decidedly proso- 
branchiate ; and similar variations, as I have shown, occur among the Pteropoda. 
The Archetypal Alimentary Canal consists of — ], lips; 2, jaws; 3, buccal mass 
and tongue ; 4, oesophagus ; 5, crop ; 6 , stomach or gizzard ; 7 , pyloric appendage ; 
8, intestine; 9, glandular appendages. I wish here merely to draw attention to some 
peculiarities of the third and the seventh organs in this list, which have not, I think, 
been hitherto sufficiently noted. 
Of the Structure of the Buccal mass and Tongue (Plate V. figs. 12, 13, 14, 15). — 
Although the structure of the “tongue” of the Mollusca has been very elaborately 
investigated, its mechanism appears to me to have been hardly at all understood. 
Cuvier, who first described this structure in Buccinuni, thought that the buccal 
cartilages were the chief agents in moving the tongue. He considered the ‘tongue- 
plate’ to be passive, and that its movements depended upon the protraction, retrac- 
tion, divergence or approximation of the cartilages -f-. 
This idea is still further carried out in the Lecons d’Anatomie Comparee (2nd 
ed. t. V. p. 15-25), where the cartilages are compared to rudimentary jaws, though a 
little consideration would have shown the jaws to be represented by other organs in 
some of the instances quoted. 
Subsequent writers either coincide in Cuvier’s view, or substitute for it some 
vague notion of licking or rasping; so Osler:|: and Troschel§. 
* The “ vena cava superior” of Cephalopoda answers to the very short trunk formed by the union of the 
two afferent branchial trunks in Aphjsia, &c., which, as I'eceiving the veins of the foot, correspond with the 
venous circlet at the base of the arms. 
t M6m. sur les Mollusques, Grand Buccin, p. 9. 
§ WiEG. Arch. 1836. 
I Philosophical Transactions, 1832. 
