XI PHYLUM MOLLUSCA 293 



secondary gills — vascular processes of the wall of the 

 mantle-cavity. 



Fig. 180. — Pulmonary cavity and related parts in a slug (Limax). anrt, aorta; 

 aur, auricle; urp/i, nephridium; pen'c, pericardium, laid open; pul. ap, pul- 

 monary aperture; pul. 7 1 , pulmonary vein with its ramifications; red, rectum; 

 Mr, ureter; z'cnt, ventricle. (After Pelseneer.J 



Digestive Organs. — In many Gastropods there is a long 

 proboscis capable of being everted and retracted, at the 

 extremity of which the mouth is placed. A single curved 

 horny jaw lies on the roof of the buccal cavity in the Pul- 

 monata; in most marine Gastropoda the place of this is 

 taken by two lateral pieces. 



A characteristic feature of the alimentary canal of the 

 Gastropoda, which, however, they share with some Amphi- 

 neura and with the Cephalopoda, is the possession of an 

 odontophore and radula, situated in a thick-walled chamber, 

 the buccal cavity, into which the mouth opens. From the 

 floor of the cavity rises an elevation, the odontophore, which 

 is somewhat elongated in the direction of the long axis of 

 the body and compressed laterally. Over the summit of 

 the odontophore runs longitudinally a narrow strap-like 

 body, the radula or lingual ribbon (Fig. 181, rad), beset 



