CIRCULATION AND RESPIRATION OF MOLLUSCA. 



335 



glands, and organs of mastication. There is frequently a com- 

 plete gizzard, or muscular stomach, for the reduction of the 

 food, when this is not accomplished in the mouth ; and the 

 intestinal tube is often of considerable length, and much convo- 

 luted, or rolled together. The blood is colourless or nearly so ; 

 and circulates, in all Mollusks, in a regular system of arteries 

 and veins, issuing from a heart, which is usually muscular or 

 nearly so, and possessed of two cavities, one of them a receiving 

 cavity or auricle, and the other an impelling cavity, or ventricle 



(ANIM. PHYSIOL. 257). 

 The accompanying figure 

 will give an idea of the usual 

 mode in which the Circula- 

 tion is carried on in this sub- 

 kingdom. The blood (which 

 has returned from the gills 

 in an aerated state) is pro- 

 pelled, by the ventricle, , 

 through the main systemic 

 artery, b ; after passing 

 through the capillaries of 

 the system, it is collected 

 by the systemic veins, /, 

 into a large trunk, which 

 again subdivides into the 

 branchial arteries, g\ these 

 convey the blood, now ren- 



dered VenOUS, to the gills, , 



* . . . 



Where it IS aerated ; and 



a ft er returning thence by the 

 branchial veins, 6?, it enters the auricle, c, whence it passes 

 again into the ventricle, a. The Respiration of Mollusks is almost 

 always aquatic ; being carried on by the aid of gills, which expose 

 a large surface of blood to the water at once. These gills are 

 sometimes altogether exterior to the body (Fig. 679) ; sometimes 

 they are enclosed between folds of the mantle (Fig. 674) ; and 

 sometimes the respiratory surface is altogether internal. In this 



d, ventricle ; ft, main artery or aorta ; c, auri- 

 cle; Abranchial veins ; e, vessels of the gills; 

 /, systemic veins ; g, branchial arteries ; h, 



tentacuia. 



