OF HELIX. 205 



blood, and the vessels which carry the circulating fluid through 

 the body. The heart lies obliquely across the mid-line of the 

 body, considerably to the left of the forward bend of the intes- 

 tine, and close to the surface of the back. From one cavity of 

 the heart, i. e., the ventricle, (A 1 ,) the blood is expelled into the 

 arteries, (GO, ao 1 , ao 2 ,) which branch through all parts of the body. 

 From the minute tips of the arteries the fluid then passes into 

 return channels, the veins, one part of which unite in a vessel 

 (hv] which goes direct to the auricle, (/*,) and another set of 

 them carries the blood to the net- work of vessels which branch 

 through the lung. After being aerated, the blood passes from the 

 smaller vessels into a single larger one, (pv,) which empties into 

 the receiving chamber, the auricle, (A,) of the heart. By the con- 

 tractions of the latter the blood is thrown into the ventricle, (A 1 ,) 

 and thence goes out, and circulates as I have just described. 



The nervous system is concentrated chiefly about the head, but 

 sends off branches to various parts of the body. It consists of 

 a double ganglion, (sg-,) the brain so-called, which lies just above 

 and across the throat, and of a still larger and broader double 

 nervous mass (g-, g- 1 ) which rests beneath the throat, and trans- 

 verse to the axis of the body. The upper and lower pair are 

 connected with each other by double or triple nervous threads, 

 (c,) which form a collar as they pass from above downwards and 

 backwards on each side of the throat. From the upper pair 

 nerves pass forward to the feelers, (t, t\) and backwards (og], on 

 the right and left, over the stomach (st). The great nervous 

 trunks (w, ng-) originate one from each of the lower pair (g-) of 

 ganglions in the head, and pass backwards along the sides of 

 the body, in nearly unbroken continuity, and with gradually 

 diminishing diameter, until they vanish in the skin and muscles 

 of the creeping disc. Where they branch there is more or less 

 of a thickening, (ng;) but scarcely deserving the name of a 

 ganglion. 



The reproductive system (figs. 122, 123, ov, ou 1 , ma, o) lies princi- 

 pally on the right side. Its essential part, (ov,) that in which the 

 eggs are developed, is deep within the spiral portion of the shell, 



