156 



BULLETIX OF THE BUREAU OF FISHERIES. 



abundance. Becoming laden with the waste products of metabolism, the blood accumu- 

 lates in the veins and sinuses which, for the most part, lie on the inner walls of the mantle 

 and superficial parts of the body where a continuous flow of water is maintained over 

 them by the cilia of the gills. This allows a ready interchange of gases with the sea 

 water, whereby oxygen is absorbed by the blood and carbon dioxide eliminated. This 

 process is continued further in the gills and plicate canals, as will be described below. 



AB p 



138 



Fig. 138. — Diagram of the circulatory system of Mylilus edulis. Au, auricle; AB, aortic bulb; ABV. afferent branchial 

 vein; EBV , efferent branchial vein; LI', longitudinal vein; NV, uephridial veins; OV , oblique vein; P, pericardium; PIC, 

 plicate canals; PV, pallial blood vessels; R, rectum; RC, renipericardial canal; V, ventricle; IT, visceral blood vessels. 



The return of the blood to the heart may take place through several channels. 

 Most of the blood from the visceral organs — stomach, intestines, liver, etc. — is discharged 

 into the lacunar system of vessels in the kidney. The blood from the mantle may flow 

 in small part into the posterior longitudinal vein and from thence be carried directly 

 to the heart without penetrating the kidney tissue. Most of the blood from the mantle, 

 however, passes into the kidney by way of the plicate canals, which have been mentioned 

 before as a compact series of thin-walled ribbonlike organs extending the length of the 

 mantle just below the roots of the gills (fig. 134, EPIC, and fig. 135, I PIC, p. 153). 

 These canals contain a spongy reticulum of elastic fibers and externally are covered with 



