CIRCULATORY SYSTEM. 137 



The radula with the cushions and the muscles 

 together make up the odontophore. 



Dissect out the radula ; boil in 5 per cent, caustic potash ; 

 and mount it on the slide in a drop of water or glycerin, with 

 its toothed surface upward. Examine it with low and high 

 powers. 



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

 on its posterior surface, against which the odonto- 

 phore works. 



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

 scope. 



VIII. THE CIRCULATORY SYSTEM. 



The dissection of the circulatory system must be performed 

 on a second snail. 



It is greatly facilitated by injecting the vessels with 

 a coloured fluid. The arteries should be injected from the 

 ventricle, and the pulmonary vein and its branches from the 

 auricle. The venous system must be injected from several 

 places ; the right pulmonary sinus is easily injected both 

 forwards and backwards, and the pedal sinuses may also be 



1. The arterial system. 



From the ventricle arises a single large vessel, 

 the aorta. This gives off almost at once a large 

 visceral artery, which runs along the upper surface 

 of the liver to the top of the spiral, supplying both 

 lobes of the liver, the intestine, and the reproductive 

 organs. 



The aorta itself runs forwards along the right 

 side of the crop, between it and the common duct, 

 giving off large branches to the salivary glands and 

 body-wall. It then continues its course forwards 

 along the under surface of the oesophagus ; pierces 

 the sub-oesophageal ganglionic mass, passing between 

 the pedal and viscero-pleural ganglia ; gives off a 



