234 OILMAN A. DREW. 



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) also food materials, and gain from 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 products and to 

 gain oxygen. Inasmuch as the heart provides for but a single 

 circulation it is necessary that the capillaries 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 properly 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 oxygen 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 gills 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. 



NERVOUS SYSTEM. 



The three pairs of ganglia that are usually found in lamelli- 

 branchs are present in this form, but they differ greatly in size and 

 they are not all placed in the usual positions. 



