76 COMPARATIVE ANATOMY 



of the gonad, probably also by the glandular part of the ex- 

 cretory organ. 



The upper whorls of the visceral hump are almost entirely 

 occupied by the lobes of a very large pair of digestive glands, 

 but they also contain several coils of the gut and parts of the 

 reproductive system. The mouth opens into a rather large 

 pharynx, the ventral wall of which bears the rasping organ or 

 odontophore. The pharynx is succeeded by an oesophagus, 

 which dilates to form a large thin-walled crop, its upper part 

 lying in the first whorl of the visceral mass. The crop is suc- 

 ceeded by an intestine, which takes several turns in the spire, 

 and eventually passes forward as the rectum and opens at the 

 anus. The digestive gland is paired, and its ducts open into 

 an enlarged section of the intestine often called the " stomach." 

 The snail is also provided with a pair of salivary glands, large 

 white lobed masses attached to the sides of the crop with 

 slender ducts running forward along the sides and opening 

 into the pharynx. 



The odontophore in the floor of the pharynx is a very char- 

 acteristic organ, and requires a special description. The pos- 

 terior part of the floor of the pharynx is produced backwards 

 into a hollow finger-shaped diverticulum, the cavity of which is 

 lined by an epithelium continuous with that of the pharynx. 

 The epithelial cells lining the ventral wall of the diverticulum 

 secrete a chitinous band, armed on its upper surface with a 

 number of minute rasp-like teeth arranged in transverse rows. 

 The chitinous band is called the radula, and the diverticulum 

 the radula-sac. In front of the radula-sac the floor of the 

 pharynx is thickened so that it projects like a cushion into the 

 cavity of the pharynx, and the radular ribbon is continued 

 forwards over the surface of the cushion. In the substance of 

 the cushion there is a piece of cartilage firmly attached to the 

 surface on which the radula rests. This cartilage can be moved 

 backwards and forwards by special muscles, and the whole 

 apparatus can be moved forwards into the mouth opening and 

 the radula made to move to and fro with a rasping action by 

 the alternate action of the anterior and posterior muscles at- 

 tached to the cartilage. The radula works against a so-called 

 jaw, which is really a chitinous curved bar traversing the upper 

 part of the oral aperture, with a ridged posterior surface. As 

 the radula is worn away in front it is replaced by growth from 



