Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/254

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Thirdly, It remains to consider the effect of the air, supposing it to be retained continually within the chambers, at the bottom of the sea. Here, if the position of the moving animal be beneath the mouth of the shell, like that of a snail as it crawls along the ground, the air within the chambers would maintain the shell, buoyant, and floating over the body of the animal in a vertical position, with little or no muscular exertion, and leave the creature at ease to regulate the movements of its tentacula (p) in crawling and seizing its prey.

cardium, excepting when it is forced into and retained in the siphuncle, by muscular pressure, during the contraction of the arms and body closed up within the shell. When these are expanded, either on the surface, or at the bottom of the sea, the water will have free access to the branchiæ, and the movements of the heart will proceed freely in the distended pericardium; which will be emptied of its fluid at those times only, when the body is closed, and the access of water to the branchiæ consequently impeded.

The following experiments show that the weight of fluid requisite to be added to the shell of a Nautilus, in order to make it sink, is about half an ounce.

I took two perfect shells of a Nautilus Pompilius, each weighing about six ounces and a half in air, and measuring about seven inches across their largest diameter; and having stopped the siphuncle with wax, I found that each shell, when placed in fresh water, required the weight of a few grains more than an ounce to make it sink. As the shell, when attached to the living animal, was probably a quarter of an ounce heavier than these dry dead shells, and the specific gravity of the body of the animal may have exceeded that of water to the amount of another quarter of an ounce, there remains about half an ounce, for the weight of fluid which being introduced into the siphuncle, would cause the shell to sink; and this quantity seems well proportioned to the capacity both of the pericardium, and of the distended siphuncle.