TORSION 267 



180°, and the anus comes to lie just behind the mouth. This is 

 torsion ; the dorsal portion then becomes spirally coiled to fit 

 the shell. Torsion is a characteristic and peculiar feature of the 

 Gastropoda (the class to which snails belong) and is developed to 

 different degrees in the various orders. 



NUTRITION 



Snails feed on leaves, out of which they cut pieces by means 

 of a toothed chitinous tongue or radula (Fig. 196), which works 



tentac/es 



jaw 



mouth 



radula 



cartilage supporting radu/a 



radula sac 



oesophagus 



=5 retractor muscle 

 of pharynx 



pedal gland— —Y^'"'^-^^ ~ 

 foot ^•'""^ =-" 



L 10 L20 



^^ /4 







Fig. ig6.— Helix pomatia. — From Thomson. 



A, Diagrammatic section of head and buccal mass, showing position of radula and jaw ; B, portion of 

 four rows of teeth from radula ( x 40), each horizontal row contains about 160 teeth and the radula 

 has approximately 160 horizontal rows ; c, central tooth of row; C, three radula teeth (x 170) ; 

 c, central, l 10, L 20, tenth and twentieth lateral. 



against a crescentic jawplate on the roof of the mouth. The radula 

 is formed in a radula sac, from which it grows as its front part is 

 worn away. On the dorsal surface of the buccal cavity open two 

 large buccal glands, the secretion of which is a lubricant. From 

 the buccal cavity a short gullet leads back and expands into a 

 large crop, on the dorsal surface of which lie the buccal glands. 

 After the crop comes a short stomach, and then a longer intestine, 

 which is coiled in the dorsal hump and then swings forward on 

 the right side as the rectum to open at the anus behind the mouth. 

 Surrounding the stomach and intestine, and occupying much of 

 the visceral hump, is a structure often called liver, but more 

 accurately known as the digestive diverticula. There are two 

 lobes, each with its own duct opening into the stomach. Particles 

 of food are carried into the diverticula, and in them most of the 



