IV. GASTEROPODA. 



373 



f erred to the right and vice versa. With this there is a tendency 

 to asymmetry and the loss of the organs (usually of the primitively 

 left side). When the nervous system takes part in the twisting 

 a notable crossing of the cerebrovisceral commissures takes place, 

 known as streptoneury or chiastoneury (fig. 364, c). 



The alimentary canal begins with a muscular region which in 

 some groups is developed into a large protrusible proboscis (fig. 

 365). The pharynx, which follows, contains the tongue, a ventral 

 ridge supported by one or more cartilages and covered by a cutic- 

 ular layer, the radula or lingual ribbon (odondophore). The upper 



surface of the radula is armed 

 with sharp, backwardly di- 

 rected teeth (fig. 366) which, 

 are usually arranged in trans- 

 verse and longitudinal rows,, 

 but which vary so in num- 

 ber, form, size, and arrange- 

 ment that they are of value in 

 classification. Although the 

 radula covers the tongue, it is 



x r 



FIG. 365. 



FIG. 366. 



FIG. 365. Pyrula tuba, male. (After Souleyet.) The mantle has been cut on the 

 right side and turned to the left, reversing the pallial organs, a, anus ; c, ctenid- 

 ium ; cm, columellar muscle ; /, foot ; 7i, heart in pericardium ; i, intestine ; I, 

 liver; 777, mantle; m/, floor of mantle cavity; ?7, nephridium; ns, opening of 

 nephridium; o, osph radium ; p, proboscis; pe, penis; t, testes; v, vas deferens 

 cut in two. 



FIG. 366. Pharyngeal region of Helix pomatia. A, side view ; B, section, m, muscle; 

 oe, oesophagus ; r, radula ; rs, radula sac ; sp, salivary duct ; z, lingual cartilage. 



formed in the radula sac, which lies behind the tongue. 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 carnivorous forms). 



The rest of the alimentary canal is convoluted, the anus being 



