BIVALVES. 
15 
The circulatory apparatus consists of a heart 
composed of two auricles and a ventricle, and 
may be readily discovered, as it is situated in the 
middle portion of the back of the animal, under 
the hinge; its beatings, which are about six or 
eight in a minute, are easily seen under the large 
bag, or pericardium, containing the heart. The 
heart continues to beat for a long time after the 
valves have been opened. The blood of shell¬ 
fish is white, or nearly colourless; and so essential 
is the red character of blood deemed by the 
vulgar, that it appears to them little less than 
an abuse of language to apply the term to the 
nearly colourless fluid of the mussel; but it 
possesses all the essential properties of blood, 
flows in a similar circle of vessels, and answers 
the same purposes in the system. The blood is 
propelled by the contraction of the ventricle into 
the arteries, and after supplying the waste of 
the body, is collected as venous blood by the 
veins from the capillary extremities; and after 
becoming aerated in the gills, is received by the 
gill-veins and convoyed as arterial blood to the 
auricles; from thence it passes into the ventricle, 
to be again distributed through the whole system. 
The respiratory organs are gills, consisting of 
two membranous plates, attached to each lobe of 
the mantle; each plate consists of two folds of 
its membrane, and is pierced by innumerable 
