OF THE ODONTOPHORB IN CEETAIN MOLLUSCA. 487 



infraradular sheet is not prevented from sliding a little way over the apex of the carti- 

 lages, and bearing the radula with it. And thus the movement, mainly due to the 

 protraction and retraction, the apical bending, and the approximation and divarication 

 of the cartilages by means of their own muscles and those of the infraradular sheet, is 

 complicated with a less important factor — a slight sliding of the radula over the apex 

 of the cartilages. (In this respect, though not in general structure, the odontophore of 

 Patella may be less specialized than that of Lolirjo.) This hypothetical interpretation 

 I have been able fully to verify by simply watching the action in live Limpets turned 

 over on their backs. 



The complex apparatus of Buccinum (figs. 24-34) will now present little difficulty. 

 The single pair of cartilages are united and curved upwards at the apex as usual, and 

 connected dorsally and ventrally by transverse fibres {dtf, vtf). As usual, too, this 

 apex is slender and sharp, bending upwards when a slight pull is given to the radular 

 sheath {rs) to imitate contraction of its muscles. The infraradular sheet is much 

 reduced, and at first sight appears to be absent ; but, as may be seen by scraping the 

 lower surface of the radula, or by taking transverse sections, it is closely attached to 

 this, and is hence not drawn separately in figs. 29 & 34. The sheet is attached to the 

 walls of the mouth on the ventral side about twice as far down as on the dorsal (figs. 

 26, 28, 29, & irs, 33), where it is even with the opening of the gullet. Thus a slight 

 contraction of the highly differentiated longitudinal dorsal radular muscles must bend 

 the weak cartilages upwards at the latter comparatively fixed point, their sulcus being 

 deepened partly by the dorsal transverse fibres (dtf), aided by the slips which pass 

 obliquely from the radular sheath to the cartilages {rs, figs. 26 & 33). The return is 

 effected by the ventral longitudinal muscles {vis), probably assisted by the contraction 

 of the transverse fibres {vtf) which connect the cartilages on the ventral side. 



Little of that sliding movement over the apex of the cartilages which we saw in 

 Patella can here take place, owing partly to the weakness and curvature in two planes 

 of the cartilages, partly to the sharpness of their apex, eminently unfitting it for a 

 pulley-block, partly to the slight fixed flexure of the radula and its want of pliability, 

 and largely also to the attachment of the infraradular membrane to the sides of the 

 mouth all round (figs. 26, 27, 28), which thus fixes the radula very steadily over the 

 cartilages. Some little yielding m«^ take place; but it must be evident, from the above 

 considerations, that the movements of the radula are similar to, and dependent upon, 

 that licking action impressed upon the buccal cartilages in the way we have seen. 



Thus the explanation here put forward has something more in common with that of 

 Cuvier\ " that the tongue-plate is essentially passive, and that its movements depend 

 upon the protraction, retraction, divergence, and approximation of the cartilages " 

 (though, of course, these have nothing to do with rudimentary jaws), than with the 

 later theory proposed by Professor Huxley 2, who, chiefly from the unquestionable fact 



' ' M^moires sur les Mollusqiies.' See also Lacaze-Duthiers ou Denlalium, Ann. Se. Nat. 1856. 

 • Huxley, Phil. Trans. 18.53, "On the Morphology of the Cephalous Molhisca." 



30 2 



