OF THE ODONTOPHORE IN CERTAIN 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 Loligo.) 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 Buecinum (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, vif). 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 (v/s), probably assisted by the contraction 
of the transverse fibres (vt/) 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 fora 
pulley-block, partly to the slight fixed flexure of the radula and its want of phiability, 
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 may 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”, who, chiefly from the unquestionable fact 
* «Mémoires sur les Mollusques.’ See also Lacaze-Duthiers on Dentalium, Ann. Sc. Nat. 1856. 
* Huxley, Phil. Trans. 1853, “On the Morphology of the Cephalous Mollusca.” 
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