THE RIVER MUSSEL. 151 



2. A circulatory system, comprising (1) veins which col- 

 lect blood from all the tissues and convey it to the gills ; 

 (2) other (branchial) veins, which convey the aerated 

 blood from the gills to the heart ; (3) auricles, which 

 receive the blood from the gills, and, contracting, pass it 

 on into (4) the ventricle, which in turn contracts, and 

 drives it through (5) the arteries, out into the tissues 

 again. 



Four important changes take place in the blood during 

 its course through the body : 



(a) It is enriched by the digested food, which it 

 receives directly through the walls of the alimentary 

 canal. 



(6) In the tissues the cells with which it comes in con- 

 tact take from it whatever material they need for growth, 

 and return to it their waste products of oxidation. 



(<?) On its return to the gills, part of the blood passes 

 through the renal organ, where nitrogenous waste products 

 are taken from it for excretion. 



(d) In its passage through the gills it loses waste car- 

 bonic acid, and absorbs oxygen. 



3. A respiratory system, comprising the gills, with 

 their numerous small blood vessels. 



4. An aquiferous system, comprising the siphons and 

 the ciliated passageways through the gills. 



5. An excretory system, of which the renal organs and 

 the gills comprise the most important part. 



Metabolism is in this, as in all other animals, performed 

 by the individual cells. 



Certain of the mantle cells take calcium carbonate from 

 the blood, and deposit it as a precipitate, thus forming the 

 mineral part of the shell. The soft inner part of the 

 mantle thus continues through life to deposit on the inside 

 of the shell a layer of pearl. If a foreign body get into 

 the substance of the mantle, or get between the mantle 



