294 



MANUAL OF ZOOLOGY 



with numerous minute horny or siliceous teeth arranged in 

 transverse row?. Posteriorly this toothed ribbon extends 

 into a narrow curved pouch, the radular sac (Fig. 181, rad. 

 sac), extending backwards from the posterior and lower 

 aspect of the buccal cavity. Anteriorly it does not extend 

 beyond the odontophore prominence. The latter contains 

 cartilages (cart), serving for the support of the whole appa- 

 ratus, and is capable of being extruded, with the radula 

 which it bears, through the opening of the mouth by the 

 contraction of sets of protractor muscular fibres. Inserted 



rcu£ bod.ca.v 



JO.W 



ca-r't. 



racL.. 



Fig. 181. — Triton nodiferus. Diagrammatic longitudinal vertical section of buccal 

 cavity, bod. cav, body cavity; cart, cartilage of odontophore; jaw, right jaw; 

 ass, cesophagus; rad, radula; rad. sac, radula sac. 



into the radula itself are sets of bands of muscular fibres by 

 which it can be drawn backwards and forwards over the 

 odontophore as over a pulley, the effect being a rasping of 

 any hard substance against which it is pressed. The entire 

 buccal cavity is capable of being drawn forwards towards 

 the mouth opening, or backwards into the introvert, by the 

 contraction of strands of muscular fibres passing from its 

 waU to the wall of the body. 



The heart is enclosed, as in the fresh-water mussel, in a 



