CIRCULATION AND RESPIRATION OF MOLLUSCA. 



335 



-A.-..& 



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, a, 

 through the main systemic 

 artery, I ; 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- 



Fm. 539. CIRCULATING APPARATUS OF DORIS.' 7 



a, ventricle; b, main artery or aorta ; c, auri- dered VCHOUS, to the gills, , 



cle ; d, branchial veins ; e, vessels of the gills 5 i , ' , j 



/, systemic veins ; g, branchial arteries ; h, Where it IS aerated ; and 



tentacuia. after returning thence by the 



branchial veins, d, 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. 569) ; sometimes 

 they are inclosed between folds of the mantle (Fig. 572) ; and 

 sometimes the respiratory surface is altogether internal. In this 



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