MOLLUSC A. 21 



large transparent one, the ventricle, and two smaller, 

 brownish-looking bags, the auricles, only one of which, 

 however, is visible from the side. (Figs. 12, \^,vr, au.) 



First, the auricles are distended with blood, — a bluish, 

 milky-looking jfluid, which is received from the gills 

 and mantle. This is forced, by the contraction of the 

 walls of the auricles, into the ventricle, which swells up 

 to accommodate the influx of blood, and immediately 

 thereafter contracts, forcing the contents out into the 

 larger arteries of the body (Figs. 2, 12, 15). The 

 principal vessel of the arterial system may be seen 

 issuing from the ventricle, and penetrating forwards 

 directly into the soft body beyond, while another passes 

 backwards along the inner side of the intestine on to 

 the inner surface of the large muscle {ad), which it 

 enters and supplies with blood. 



On the way it also branches laterally, giving blood to 

 the whole surface of the intestine and mantle in the 

 regions on either side of the intestine in the neighbor- 

 hood of and behind the muscle (Fig. 12, iai) . 



The great artery, after it enters the body, throws off 

 three branches to the ventral side (Fig. 12). The first of 

 these passes directly to the viscera on the lower side of the 

 body ; the next passes to the intestine, by means of a pos- 

 terior branch, and then, passing onward towards the great 

 venous sinus and under the intestine again, gives off two 

 branches, one reaching anteriorly to the liver and genital 

 organs, which is cut in Fig. 12, and the other posteriorly 

 to the intestine and genital organs in the region below the 

 great muscle ; the third branch, which is severed in Fig. 12, 

 goes to the stomach, intestine, liver, etc., on the right side 

 of the visceral cavity. After giving off these branches, the 

 aorta passes forward through the loop of the intestine (/), 



