MOLLUSCA—THE ALIMENTARY CANAL 



181 



The ventral and lateral walls of the pharynx are thickened and 

 very muscular. On the floor of the cavity rises a tough longitudinal 

 muscular cushion, the tongue. Its surface, Avhich projects into the 



9 1 s "l ^ 



Fir,. 153.— Longitudinal section (not quite median) through the snout of a Prosobranchiate, 

 to illustrate the pharyngeal apparatus. 1, Dorsal wall of the head; 2, mouth; 3, Jaw; 4, 

 radula ; 5, lingual cartilage ; 0, muscular wall of the pharynx ; 7, muscles attached at one end to the 

 pharynx and at the otlier to the ventral wall of the head (S) ; 9, cavity of the head ; 10, radular 

 sheath ; 11, oesophagus ; 12, aperture of the salivary gland ; 13, infolding behind the radular sheath. 



pharyngeal cavity, is covered by a rough cuticle consisting of chitin 

 (or conchyolin ?) ; on this basal membrane are found very numerous 

 hard chitinous teeth, often many thousands, arranged in close transverse 



Fig. 154.— Median longitudinal section through the anterior part of the body of Helix 

 (after Howes), a, CEsophagus ; rd, radular sheath ; nc, cerebral ganglion ; sU, aperture of the 

 salivary gland ; oc, muscle mass in the ventral pharyngeal wall; rd, radula ; hj, upper jaw ; li, lo, 

 lips of the oral aperture ; im, phai-yngeal muscles ; rm.j, retractors of the pharynx ; pgl, pedal 

 gland. 



and longitudinal rows. The basal membrane and the teeth together 

 form the radula of the tongue. 



The anterior end of the tongue projects freely into the pharyngeal 

 cavity, the radula bending down over this end so as to cover for a 



