230 HUGH WATSON. 



of the kidney, being near tlie suture of the shell, would I'einain 

 in its original position ; and accordingly we find that in the 

 Oleacinidae the kidney is obliquely lengthened in a very 

 characteristic manner. 



The cerebral, buccal, and ventral ganglia would be pushed 

 further apart by the growth of the buccal mass and odonto- 

 phore, and thus we find that in many of the carnivorous forms 

 the cerebro-buccal, cei'ebro-pedal, and cerebro-pleural con- 

 nectives are unusually long (see pp. 141, 146). 



The extrinsic buccal retractors would become strongly 

 developed at the same time as the intrinsic muscles of the 

 odontophore. So long as they sprang fi'om the columellar 

 muscle, they would undergo little change beyond an increase 

 in their thickness, although I have found that their anterior 

 ends tend to split up into a number of separate strands in the 

 Rhytididte. But in those forms in which the shell is 

 degenerate and the skin is taking its place, we frequently find 

 that the retractors of the odontophore arise from a large area 

 of the integument; as, for example, in Apera gibbon si, 

 Testacella haliotidea Drajy., and the Trigonochla- 

 minse (see pp. 172, 173). 



A jaw becomes superfluous in a snail or slug in which the 

 radula is protruded far beyond it, and Simroth ^ has shown 

 that it w^ould be a positive disadvantage in a vermivorous 

 form, because if it bit off pieces from its prey, the latter 

 would escape. Accordingly the jaw is absent or extremely 

 degenerate in iiearly all carnivorous genera excepting 

 Plutonia, in which Simroth states that the sharp edge is 

 covered by a softer downward growth. 



As in other carnivorous animals, the digestive region of the 

 alimentary canal becomes reduced in size. This is most 

 apparent in the slugs, for in these the pressure of the organs 

 which formerly occupied the visceral hump tends to diminish 

 anything in the body-cavity that is unnecessarily large. In 

 A top us and Apera the true stomach has almost completely 

 disappeared, the crop passing straight into the intestine ; and 

 1 Op. cit.. p. 113. 



