294 MANUAL OF ZOOLOGY SECT. 



it bears, through the opening of the mouth by the contrac- 

 tion of sets of protractor muscular fibres. Inserted 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 



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FIG. 169. Triton nodifents. Diagrammatic longitudinal vertical section of buccal 

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

 ?j. oesophagus ; rod. radula ; rad. sac. radula sac. 



contraction of strands of muscular fibres passing from its 

 wall to the wall of the body. 



The heart is enclosed, as in the Fresh-water Mussel, in 

 a cavity the pericardium. It consists, in nearly all cases, 

 of only two chambers, an auricle and a ventricle. 



The nervous system and organs of special sense are in 

 most Gastropoda more highly developed than they are in 

 the Fresh-water Mussel. There are. distinct cerebral and 

 pleural, as well as pedal and visceral ganglia. Well- 

 developed eyes are present in the majority, and there are 



