420 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



brane, some being inserted into its posterior and lateral por- 

 tions, and others into its anterior extremit}', after it has 

 turned over the anterior extremities of the principal cartilages. 



Fig. 119.— Buccinum undatum.—A, radula. B, one of the transverse rows of teeth ; 

 a, anterior, b, posterior end ; c, central, I, lateral teeth. (After Woodward, '"Man- 

 ual of the Mollusca.") 



At 



Fig. 120.— A, TrocJius cinerarivs; the median tooth and the teeth of the right half 

 of one row of the radula. B, Ci/prcea, Europoea, one row of teeth of the radula. 

 (Woodward, ibid.) 



Certain of the muscular bundles are also attached to the fore- 

 part of the odontophoral cartilages themselves. The con- 

 traction of these muscles must tend to cause the subradular 

 membrane, and with it the radula, to travel backward and 

 forward over the ends of the cartilages in the fashion of a 

 chain-saw, and thus to rasp any body against which the teeth 

 may be applied. When undisturbed, the radula is concave 

 from side to side, and the teeth of the lateral series, being 

 perpendicular to the surface to which they are attached, are 

 inclined inward toward one another. But when the intrinsic 

 muscles come into action, the radula, as it passes over the 

 ends of the cartilages, becomes flattened, and the lateral teeth 

 are consequently erected or divaricated. The extrinsic mus- 

 cles pass from the odontophore to the lateral walls of the 

 head, and protract or retract the whole apparatus. They 



