IV. GASTEROPODA. 



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

 to asymmetry and tlie 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 talies place, 

 known as streptoneury or chiastoneury. 



The alimentary canal begins with a muscular region which in 

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

 305). 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. 306) Avhich 

 are usually arranged in trans- 

 verse and longitudinal rows, 

 but which vary so in num- 

 ber, form, size, and arrange- 

 ment tliat they are of value in 

 classification. Although the 

 radula covers the tongue, it is 



z r c 



Fig. 365. 



Fig. 368. 



Fio. 36.5.-Pi/n(!a tuha, male. (After Souloyet.) The mantle has ''een cut on the 

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

 ium; cm, columellar muscle; /, foot; /., heart in pencardium ; t, intest me (, 

 liver; m\ mantle; mf. floor of mantle cavity; „, nephr.dium '•-■. oPe'"ns °t 

 nephridium; o, osphradium ; /-, proboscis; pc, penis; (, testes, i\ vas deferens 



F.G'swi-Phii'ryngeal region of HHix pomatia. A. side ^i*^^ J «'f f;t;™al'ca?t."ugc?' 

 n,:. (esophagus ; r, radula ; r,s, radula sac ; up, salivary duct , z, lingual cartilage. 



formed in the radula sac, which lies behind the tongue. ^ From 

 this it o-rows 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 



