CH. VII] SNAILS AND SLUGS 213 



spotted with black) a small shell is present. The carnivorous 

 slugs of the genus Testacella carry on the hinder part of 

 the body a cap -like shell with evident traces of spiral 

 growth. It is thus clear that slugs are descended from 

 ancestors that possessed well-developed shells within 

 which the animal could be retracted. 



The locomotion of these Molluscs is proverbially slow. 

 Slugs are less tardy than snails and have been timed to 

 travel at the rate of a mile in about eight days, whereas a 

 snail (H. aspersa) would take nearly fifteen days to cover 

 the same distance 1 . The movement is a steady, gliding 

 one and is brought about partly by muscular and partly 

 by ciliary action. From the pedal gland, whose aperture 

 is just below the mouth and above the anterior end of the 

 foot, mucus is discharged on to the surface over which the 

 animal is travelling : the discharge is effected by the cilia, 

 which line the cavity of the gland, assisted by muscular 

 compression. On the smooth bed of mucus thus laid down 

 the animal glides forward, advancing by the aid of the 

 cilia which cover the sole of the foot (and the sides in 

 some slugs, e.g. Ariori) and by a series of successive waves 

 of muscular contraction and expansion flowing over the 

 sole from behind forwards. These waves may vary in 

 number from some 30 to 50 per minute. The slimy trail left 

 behind by the mollusc is marked with transverse wavy 

 ridges and furrows about a millimetre apart as the result 

 of the undulatory movements of the foot. Tracks of a 

 more permanent kind are often made by snails or slugs 

 gnawing the surface over which they are moving; such 

 1 Jeffreys (Thomas), Brit. Conch, i. 



