THE SNAIL 77 



behind, just as a finger nail is continually replaced by growth 

 forward from its bed. The chitinous teeth of the radula are 

 formed from the epithelium lining the extreme hinder end of 

 the radula sac. In the snail the radular teeth are minute, 

 directed backwards and similar, and are regularly arranged in 

 numerous transverse rows (fig. 18, B\ In many gastropods 

 the teeth of any single row differ considerably in size and shape 

 according to their position, and one may distinguish central, 

 lateral, and marginal teeth, but the teeth of successive rows are 

 always similar. It would be out of place to enter upon a de- 

 scription of the various kinds of radular teeth found in gastro- 

 poda, but they are used as a basis of classification, and are 

 therefore of some importance. 



The nervous system of Helix is much concentrated, and 

 its central parts form a collar which, when the animal is ex- 

 tended, surrounds the anterior part of the oesophagus, but 

 when the animal is retracted the pharynx is pulled back 

 through it. The nerve collar is covered by a thick coat of 

 connective tissue, which must be removed before the con- 

 stituent ganglia and nerve cords can be clearly seen, and even 

 then the subeesophageal ganglia are so closely massed together 

 that their outlines are difficult to determine. Hence it will 

 make the description clearer if we begin with the study of the 

 nerve system in Limnaa stagnalis (fig. 18, B\ In this fresh- 

 water snail there is a large pair of cerebral ganglia situated 

 above the oesophagus, and connected by a short and broad 

 cerebral commissure. Nerves are given off anteriorly to the 

 cephalic tentacles, and two nerves pass forwards on the 

 sides of the pharynx, and there enlarge to form a pair of 

 buccal ganglia, connected by a transverse commissure passing 

 under the pharynx. Posteriorly, two pairs of stout connectives 

 pass backwards from the cerebral ganglia. The lower and 

 inner pair are the cerebro-pedal connectives, and they enter a 

 large pair of pedal ganglia lying ventral to the oesophagus. The 

 pedal ganglia are united by a pedal commissure, and give off 

 stout pedal nerves to the muscles of the foot. The upper and 

 outer pair of connectives are the cerebro-pleurals. They join 

 a pair of pleural ganglia lying below the oesophagus, but above 

 and to the outside of the pedal ganglia. The pleural are united 

 to the pedal ganglia by short pleuro-pedal connectives, and 

 they further give off posteriorly a pair of nerves which, after a 



