ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 1 85 



of all MoUusks, and that prosobranchism is one result of that asym- 

 metrical development which I have endeavoured to show to be the 

 principal agent in modifying the form of these animals. A little con- 

 sideration will render it evident, that neither the neural nor the hcevial 

 flexure will have any effect in altering the position of the heart, so long 

 as the flexure occurs behind it, while either flexure will produce proso- 

 branchism, 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 opistho- 

 branchiate, while Atlanta is very decidedly prosobranchiate ; and 

 similar variations, as I have shown, occur among the Pteropoda. 



The Archetypal Alimentary C«««/ consists of — i, 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. [Plate 20] 

 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 Buccinum, 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, retraction, divergence, or approxima- 

 tion of the cartilages. 1 



This idea is still further carried out in the Legons d'Anatomie 

 Comparee (2nd ed. t. v. p. 15-25), where the cartilages are compared Jo 

 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.^ 



Middendorf, in his elaborate Monograph upon Chiton (Malaco- 

 zoologia Rossica), gives a very careful and detailed description of the 

 buccal apparatus in that Mollusk, but equally fails in rendering its 

 action clear. 



^ Mem. sur les Mollusques, Grand Biiccin, p. 9. 



'^ Philosophical Transactions, 1832. ^ wieg. Arch. 1836. 



