170 



longitudinal muscles of the " foot," the snail glides along 

 or creeps. 



Write an Account of the Alimentary and Nervous Systems 

 of Helix. 



The mouth opens into the buccal cavity, which has, on 

 the roof, a horny bar or jaw, and on the floor, a radula, 

 a chitinous ribbon set with transverse rows of minute 

 teeth. The front portion of the radula rests on a car- 

 tilaginous pad, and its hind portion lies in the tubular 

 radula-sac, where it was formed and where it is added to 

 when worn. It is worked, by muscles, against the jaw, 

 and in this way the snail rasps pieces out of leaves on 

 which it feeds. The whole mechanism is termed the odonto- 

 phore. From the buccal cavity a short gullet leads into the 

 large storing crop. Attached to the outside of the crop 

 are the two diffuse white salivary glands ; their ducts pass 

 forward along each side of the gullet and open into the 

 buccal mass. The large brown bilobed digestive gland 

 occupies the greater part of the coiled visceral hump, 

 and it surrounds the next portion of the gut, the " stomach," 

 a dilated loop into which its ducts open. This is followed 

 by the long intestine, the first portion of which is embeded 

 in the digestive gland or " liver " and is coiled ; the terminal 

 portion (rectum) has a straighter course, passing forward 

 to the anus. 



The central nervous system consists of a ring round the 

 gullet with two supra-oesophageal or cerebral ganglia and 

 a sub-oesophageal ganglionic mass, which is composed of 

 pedal and pleuro-visceral ganglia. From those ganglia, 

 nerves are given off to the head, " foot," mantle, and 

 viscera. 



How does the Snail breathe ? Describe the Heart and the 

 Course of the Circulation. 



Air passes through the pulmonary aperture into the 

 mantle-cavity or pulmonary chamber, the space between 

 the mantle and the body- wall. On the inner surface of 



