genous waste in the blood that has traversed the mantle is so 

 small that it would diminish the proportion of nitrogenous 

 waste in the blood, if this blood were added to the blood that 

 passes through the kidneys. 



The blood that goes to the general system must in its progress 

 lose a considerable portion of its oxygen, and (in all portions 

 except around the alimentary canal, where there is, of course, 

 a decided gain) food materials, and gain from the waste of the 

 tissues a considerable amount of nitrogenous and carbonaceous 

 wastes. It is then essential that such blood should go to the 

 excretory and respiratory organs to get rid of these waste prod- 

 ucts and to gain oxygen. Inasmuch as the heart provides for 

 but a single circulation it is, of course, necessary that the capil- 

 laries of these organs be traversed before the blood is returned 

 to the heart. Why it is arranged so part of the blood may 

 dodge the kidneys and be carried directly to the gills is not 

 nearly so evident. Possibly the periodically great activity of 

 the adductor muscle causes the blood to move through it so 

 rapidly that the small kidneys cannot take care of it and prop- 

 erly perform their function, and the other channel is provided 

 to carry the surplus away to the comparatively extensive gills 

 where the increased flow can be taken care of with greater ease. 

 It is, of course, essential that the amount of orygen in the blood 

 at such times shall not be reduced. It is at any rate evident 

 that there is a possibility that part of the blood that is returned 

 from the muscle, liver, etc., may not pass through the kidneys, 

 for when starch injecting mass is injected through a vessel that 

 carries blood from one of the kidneys to the gills not only are the 

 kidney and the gill injected, but part of the mass usually finds 

 its way into the adductor muscle, liver, and other organs of the 

 body. 



The rate of the heart beat is slow, and as in other lamelli- 

 branchs is, no doubt, dependent upon the temperature of the 

 animal as well as on other factors. The auricles and ventricle 

 become very greatly distended during diastole, and contract so 

 that their cavities are almost entirely obliterated in systole. 



42 



