486 . MR. P. GEDDES ON THE MECHANISM 



closely fitted by its non-dentigerous flaps. In action the powerful internal retractor 

 muscle (dim), aided by the external retractor (eclhn), must drag slightly upon the apex 

 of the radula, but mainly upon the apex of the infraradular sheet ; and both, prevented 

 from sliding over the apical edge of the framework by their sharp permanent curvature 

 in both longitudinal and transverse sections, as well as by the parallel curvature and 

 absence of firmness in the framework itself, must be pushed upwards and forwards, the 

 teeth of the radula thus scraping harshly against an object held in the jaws immediately 

 above. During this upward and forward curvature, if the dorsal transverse fibres of 

 the framework (dff) contract, they will assist the stroke, at the same time deepening 

 and narrowing its cavity. The return is effected by the elasticity of the whole 

 apparatus, assisted by the contraction of the ventral longitudinal muscles of the infra- 

 radular sheet (vis, figs. 1, 4, 5), as also by the ventral transverse fibres of the support 

 (rff, figs. 1, 4, 8). A pretty good rough idea of the action may be gained by laying 

 the open hand on the table with the palm upwards and bringing up the four finger- 

 tips till they nearly meet, the knuckles being meanwhile pushed forward along the table, 

 and then drawn backwards as the hand reopens — or even by moving the tip of the 

 tongue, as in licking, curling up its sides at the same time, to represent the sulcus. 



Attention is particularly called to the fact that the construction and adjustment of 

 each and all of the three parts of the whole mechanism — the radula, the inlraradular 

 sheet with its muscles, and the framework, in all the details above described — is such as 

 to render a strap-over-pulley motion practically impossible. 



In Patella (figs. 10-23) the framework is very highly developed, containing three 

 pairs of cartilages. The anterior pair is continuous for a short distance in front, and 

 the pointed apex, though slightly rolled downwards, is very thin and flexible. The 

 ventral pair of protractor muscles, united into a continuous sheet (vjir, fig. 14), originate 

 in the lower lip ; the postero-lateral (/pr, fig. 20) at the sides of the head. Both pairs 

 are inserted into the posterior buccal cartilages ; but while the lateral muscles are attached 

 at about the middle of these, so as to pull the whole buccal mass forward without 

 moving its parts upon each other (as also do the anterolateral pair), the lower sheet is 

 attached to the ventral edge of the cartilages (figs. 14, 20) ; and these, therefore, rock 

 upon the anterior pair at the joint, thrusting them upwards and forwards. 



The infraradular sheet fits over the anterior pair of cartilages, but is too thin to have 

 a permanent angle at the apex as in Loligo. Its ventral muscles originate in the 

 posterior buccal cartilages' ; and their contraction must push the anterior pair, and thus 

 the apex, downwards. The powerful muscles which antagonize them on the dorsal 

 side must bend the cartilages upwards at the apex through a not inconsiderable arc, 

 the sulcus at the same time deepening. It will be seen, however, that in this case the 



' These cartilages seem to be represented in Loligo by a membranous sheet similarly placed, and from which 

 the dorsal and ventral muscles of the infraradular sheet originate. 



