ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 1 87 



When the muscular expansion formed by the lower protractors is 

 removed, four or five muscular bands (fig. 12 /a) are perceived inserted 

 by their posterior extremities into the posterior and lower part of the 

 " buccal cartilages," and converging anteriorly to be inserted into the 

 lower edge of an "elastic plate." 



From the same point of origin a thick bundle of reddish fibres 

 passes up over the posterior extremity of the cartilages, and is inserted 

 into the upper edge and sides of the " elastic plate." These may be 

 called the intrinsic muscles (fig. 13 /x). 



This elastic plate (?j) is an elastic transparent membrane, broad 

 posteriorly, and narrower anteriorly, so as to be somewhat heart- 

 shaped. By its superior surface it gives attachment to the " den- 

 tigerous plate" (lamina radulse of Midd.), on which the teeth are set ; 

 inferiorly it is very smooth, and plays over the equally smooth pulley- 

 like surface afforded by the larger buccal cartilages (fig. 14). These 

 are four in number, two large and two small accessory ones (S). The 

 larger are elongated, white, cartilaginous-looking plates, excavated 

 internally, and thick and convex behind ; their inner edges are kept 

 together by strong transverse muscular fibres. Their upper edges are 

 in contact, forming the smooth surface mentioned above ; the smaller 

 seemed to be in a manner sesamoid cartilages ; they are connected 

 anteriorly with the tongue-plate and posteriorly with muscular fibres, 

 which are inserted into the larger cartilages. 



It is clear that the action of the intrinsic muscular bands (having 

 the insertions described) must be to cause the " elastic plate," and with 

 it the " dentigerous plate," to traverse over the ends of the cartilages, 

 just like a band over its pulley, the cartilages themselves being en- 

 tirely passive in the matter. The extrinsic bands, again, must serve to 

 protract the whole mass and thrust it more or less firmly against the 

 object to be acted upon. 



I have examined Buccinum, Fissurella, Doris} Aplysia, Bullcea, 

 Helix, Onckidium, Cyprcea, Pteroceras, Sigaretus, and Veriiictus, and in 

 all I have found a structure essentially similar to that here described ; 

 the difference consisting in the greater or less length of the dentigerous 

 plate, and the more or less complete development and isolation of the 

 buccal cartilages. These are the less distinct the more the tongue 

 becomes a mere organ of deglutition. Aplysia and Bullcea, for in- 

 stance, have the cartilages united and much softer than in most 

 genera. The structure of the Cephalopod tongue closely resembles 

 that of Aplysia ; and it has the further peculiarity, that the portion of 



1 See also the description of the "tongue" in this genus by ISIessrs. Hancock and 

 Embleton, loc. cit. p. 2io. 



