122 THE SNAIL 



jar as the jaw. Separate the two sides so as to expose the 

 cavity of the buccal mass. 



1. The odontophore or radula is a brownish-yellow chitinous 



riband lying on the floor of the buccal mass, and 

 having its free surface produced into an immense 

 number of minute backwardly directed teeth, arranged 

 in very regular transverse rows. 



The hinder end of the odontophore is folded lon- 

 gitudinally on itself, and lodged in a tubular sac, 

 which projects freely behind the buccal mass, and 

 within which the odontophore is formed as a cuticular 

 secretion of its epithelial lining. The anterior end is 

 opened out and rests on a cartilaginous cushion. By 

 means of protractor and retractor muscles, the whole 

 organ can be protruded from the mouth, and worked 

 backwards and forwards with a licking or rasping 

 action. As it wears away in front, it is replaced 

 by growth forwards from its hinder end. 

 Dissect out the odontophore ; clean it ; and vwunt it on a 

 slide in a drop of water or glycerine, with its toothed surface 

 upwards. Examine it with low and high powers. 



2. The jaw is a semicircular chitinous bar, strongly ridged 



on its posterior surface, against which the odontophore 

 works. 

 Dissect out the jaw and examine it with a lens or the 

 microscope. 



VIII. THE CIECULATOEY SYSTEM. 



The dissection of the circulatory system must he performed 

 on a second snail. 



It is greatly facilitated by injecting the vessels with 

 a coloured fluid. The arteries should he injected from the 

 ventricle, and the pulmonary vein and its branches from the 

 auricle. The venous system must be injected from several 

 places J the right pulmonary sinus is readily injected both 

 forwards and backwards, and the pedal sinuses may also he 

 injected. 



