IV. GASTEROPODA 



329 



pharynx, which follows, contains the tongue, a ventral ridge supported by 

 one or more cartilages and covered by the lingual ribbon or radula (odon- 

 dophore). The upper surface of the radula is armed with sharp, back- 

 wardly directed teeth (fig. 334) arranged ift transverse and longitudinal 

 rows; these vary so in numljer, form, size, and arrangement that they 

 are of value in classification. The radula is formed in the radula sac, 



which lies behind the tongue 

 (fig. 334, rs). From this it 

 grows forward like a nail over 

 its bed as fast as it is worn out 

 in front. It is opposed in eating 

 by a single median or a pair of 

 lateral jaws (lacking in carnivor- 

 ous forms). 



The rest of the alimentary 

 canal is convoluted, the anus 



Fig. 333. i-io. 334. 



Fig. 333. — Pyrula tuba, male (after Souleyet). The mantle has been cut on the 

 right side and turned to the left, reversing the paUial organs a, anus; c, ctenidium; 

 cm, columellar muscle; /, foot; h, heart in pericardium; /, intestine; /, liver; m. mantle; 

 m/, floor of mantle cavity; ii, nephridium; ns, opening of nephridium; o, osphradium; 

 p, proboscis; pe, penis; t, testes; v, vas deferens cut in two. 



Fig. 334. — Pharyngeal region of Helix pomatia. A, side view; B, section, m, 

 muscle; oe, cesophagus; r, radula; rs, radula sac; sp, salivary duct, 2, Ungual 

 cartilage. 



being usually on the right side in front, in or beside the mantle chamber 

 (figs. 337, 338, 339). Rarely it empties in the middle line behind. 

 CEsophagus, stomach, and intestine are slightly marked ofi' from each 

 other. The convolutions of the intestine are enveloped by the liver, 

 which forms the chief part of the visceral sac. A pair of salivary 

 glands empty into the pharynx, these in the Doliida: secreting a saliva 

 containing 5 per cent, of free sulphuric acid. 



The nervous system usually differs from that of other molluscs in 



