CH. VII] SNAILS AND SLUGS 217 



horny sheet covered with an immense number (about 

 15,000) of small, backwardly directed teeth; it is carried 

 upon the subradular membrane, which in turn is borne 

 upon cartilaginous pads capable of rotating through a 

 small angle. The whole is worked by a system of protractor 

 and retractor muscles. The action may be most con- 

 veniently observed in aquatic gastropods crawling upon 

 the glass of the aquarium and browsing on the microscopic 

 algae that adhere to the glass. At each bite the upper 

 lip is retracted and the jaw brought close to the glass, the 

 radula is then pushed forward and makes an upward 

 rasping stroke : by this movement the algae are torn off 

 and rasped against the jaw. The radula of Testacella is 

 capable of rapid protrusion, the prey being impaled upon 

 the teeth and drawn into the mouth, which in this case is 

 not furnished with a jaw. The anterior portions of the 

 radula are continually being worn away by friction but, as 

 continually, are replaced by fresh growths from behind. 

 The growth takes place in the radula sac, which is a 

 ventral offset from the hinder part of the mouth cavity, 

 and the whole horny sheet slides slowly forward over the 

 floor of the mouth, keeping up an unending supply of fresh 

 teeth throughout life. 



The food in the mouth is exposed to the action of the 

 secretion of the salivary glands. This fluid not only 

 moistens the food to assist its passage along the alimentary 

 canal but also converts the starches of the food into sugar 1 . 

 The lining of the canal is more or less provided with cilia, 

 whose action, combined with the peristaltic contractions of 



1 Bonardi, Boll. Sc. Pavia, 1883. 



