4 02 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



to carry away excretions. In siphonated bivalves, as 

 already seen, the currents pass down one siphon and out 

 the other. 



Circulation: Heart.— In the longitudinal section (Fig. 

 280) H is the heart, through which runs the intestine, i, 

 on its way to the vent. The heart consists of one ven- 

 tricle, and usually of two 

 auricles. The relation of 

 the heart to the gills is seen 

 in Fig. 282. The blood 

 from the ventricle is thrown 



Fig. 281. — Transverse section of 

 Anodonta : v, ventricle ; au, auri- 

 cle ; vs, venous sinus ; m, mantle ; 

 br, branchiae or gills ; /, foot ; k, 

 kidneys. The arrows show the 

 course of the circulation. 



Fig. 282. — Diagram of heart and 

 branchiae of a bivalve, viewed 

 from above : v, ventricle ; au, 

 auricle ; dr, branchiae. 



fore and aft to the tissues, thence gathered from capil- 

 laries by veinlets and emptied into several sinuses scat- 

 tered in different parts of the body. From these it is 

 taken up by the branchial vessels, which distribute it to 

 the gills, where it is aerated, and thence by branchial 

 veins to the auricles on each side and to the ventricle, 

 which again throws it in the tissues. A schematic dia- 

 gram which represents this would be similar to that used 

 for crustaceans (Fig. 279), except for the addition here 

 of the auricle, but the details of the circulation are, of 



