256 



COLLEGE ZOOLOGY 



laries and flows through these into sinuses. Veins lead from 

 these sinuses to the walls of the mantle cavity, where the blood, 

 after taking in oxygen and giving off carbon dioxide, enters the 

 pulmonary vein (Fig. 181, 13) and is carried to the single 

 auricle (12) and finally into the ventricle (11) again. 



Excretion. — The glandular kidney (Fig. 181, 8) lies near 

 the heart. Its duct, the ureter or renal duct (9), runs along beside 



the rectum and opens (10) near the 

 anus (7). 



Nervous System. — Most of the 

 nervous tissue of the snail is concen- 

 trated just back of the buccal mass 

 and forms a ring about the oesoph- 

 agus (Fig. 1S1, in black; Fig. 183). 

 There are five sets of ganglia and 

 four ganglionic swellings. The supra- 

 oesophageal or cerebral ganglia (Fig. 

 183, 4) are paired and lie above the 

 oesophagus. Nerves extend anteri- 

 orly from them, ending in the two 

 buccal ganglia (1), the two eyes, the 

 two ocular ganglionic swellings (3), the 

 Fig. 183. — Central portion two olfactory ganglionic swellings, and 



of the nervous system of Helix . . - T ... 



pomaiia. i, buccal ganglion; the mouth. Nerves called commis- 

 2, optic nerve with thickened sures connect the supra-cesophageal 



root (3) arising from the cere- ,. •„! ,1 «• i • i v 



bral ganglion ( 4 ); 5, pedal, ganglia with the ganglia which llfi 

 6, pleural, 7, parietal, 8, vis- beneath the oesophagus. Here are 



ceral ganglion. (From Lang, - . . .... . 



after Bohmig and Leuckart.) f °ur pairs of ganglia lying close to- 

 gether — the pedal (5), pleural (6), 

 parietal (7), and visceral (8). Nerves pass from them to the 

 visceral hump and the basal parts of the body. 



Sense-organs. — Both the foot and the tentacles are sensitive 

 to contact, and are liberally supplied with nerves. Each long 

 tentacle (Fig. 180, Fu.) bears an eye. These eyes are probably 

 not organs of sight, but only sensitive to light of certain intensities. 



