XIII] 



HELIX 



289 



plan as that of the snail. Where there are two gills there are 

 likewise two kidneys. Often there is no ureter, but the kidney 

 opens directly to the exterior, as in the Cuttle-fish, the Whelk 

 (Buccinum), and the Limpet (Patella). In the Cuttle-fish instead of 

 irregular spaces there are regular veins in the walls of the kidney 

 and the special excretory cells are only developed over the course 

 of these veins. 



Turning now to the digestive system of the snail we notice 

 several very interesting peculiarities. The mouth is situated in 

 front, beneath the small pair of tentacles, and there is a curved 

 horny bar, the jaw, in the roof of the mouth. Against the jaw 

 works a rasp-like tongue, called the radula, the surface of which 



FIG. 130. Inner view of right half of head of Helix, to show the arrangement 

 of the radula x 2. 



1. Mouth. 2. Horny jaw. 3. Kadula. 4. Cartilaginous piece sup- 

 porting radula. 5. Kadula sac from which radula grows. 6. Muscle 

 which retracts the buccal mass. 7. Intrinsic muscles which rotate the 

 radula. 8. Cerebral ganglion. 9. Pedal and visceral ganglia. 



10. Oesophagus. 11. Anterior tentacle. 12. Eye tentacle. 13. Orifice 

 of duct of salivary gland. 14. Mucous gland which runs along foot and 

 opens just under the mouth. 



is a horny membrane covered with myriads of minute, recurved 

 teeth. Underneath this membrane there are certain small pieces of 

 cartilage to which muscles are attached which pull the cartilages 

 and the membrane covering them alternately downwards and for- 

 wards and upwards and backwards, so that the tongue is worked 

 against the jaw. Thus the snail is enabled to tear pieces out of 

 the leaves on which it feeds (Fig. 130). A similar organ is found 

 in all Mollusca, except the bivalves or Lamellibranchiata, and the 

 number, shape and arrangement of the teeth are an important help 

 in classification. The horny membrane is continued backward into 

 s. & M. 19 



