The heart, constricted at the middle like the former silk purses of the ladies, is shown in place. The constriction separates the auricle and the ventricle. And so even an oyster has three sets of circulating organs, the heart with its double set of functions, and the arteries and veins. And this little organ beats with regular pulsations. That little auricle receives the blood from the gills, and that tiny ventricle is the vital force-pump that propels it into the arteries. "From the capillary extremities of the arteries it collects again into the veins, circulates a second time through the respiratory organ, and returns to the heart as arterial blood." The color of the oyster's blood is a pale bluish white—in fact it may be called opaline. Our oyster, then, is not a heartless thing. If you open it with care and skill, as would the naturalist, you may see and count the throbbings of its tiny heart.
In its proper place is seen the liver, which is always a large organ in the mollusca, or so-called shell-fish. It is true that this organ in the oyster secretes bile, and doubtless in large quantities. It is not probable, however, that this organ, though large, ever performs a metaphorical function, for it is very doubtful whether the oyster ever gets up the amount of emotion necessary "to stir one's bile." To the fast liver this oyster-liver is every thing. The secret is just here: this secretion of the liver is the real appetizer of the feast. This oyster-bile is both gustatory and digestive. It excites the glands of the palate and the secretions of the stomach.
The part indicated by the word muscle is the portion through which the knife, is passed when opening an oyster. In popular parlance it is sometimes called the "eye," and by some the "heart;" terms which, thus applied, are without meaning. It is the adductor muscle, and is the organ with which the oyster pulls-to its doors.
To sum up these considerations of the oyster's physiology, we see that, to the full extent of its necessities, it has distinctive sets of organs for the performance of the three classes of functions carried on in our own organization, namely, ingestion, respiration, and circulation.
The Oyster's Shell.—The toughest part of the oyster is the adductor muscle (Fig. 7). The office of this large, strong muscle is to pull-to and keep shut the great doors of the house. And a very curious bit of mechanism is subsidiary to this action. At the upper part of the cut is seen the hinge, a white spot with a dark curve below it. This dark curve is the hinge-ligament. It is a dark substance which fills up the pit or depression near the hinge. In the living animal it is wonderfully like gutta-percha—black, tough, and elastic. Let us attempt to explain its use to the oyster. Although this mollusk has a strong muscle with which to close its valves, it has not any with which to open them. Now, supposing we should take a lady's writing-desk, and, between the hinges at the back of the lids, should insert a piece of India-rubber, then should press down the lid, and turn the key; it is plain that the bolt of the lock now keeps the lid down, which could