. Comparative anatomy and physiology. 308 COMPARATIVE ANATOMY AND PHYSIOLOGY, In many cases lines of growth can be made out, and the shell may continue to increase in size for so long a time that a single valve of a Tridacna will be found to weigh a hundred and fifty-six pounds. In Nautilus the shell consists of a number of chambers (Fig. 126), each of which is larger than that which precedes it, and is formed by the animal as it increases in size; each of these chambers is separated. Fig. 126.—A Section of the Shell of the Pearly Nautilus, showing the successive Chambers occupied by the Anima
. Comparative anatomy and physiology. 308 COMPARATIVE ANATOMY AND PHYSIOLOGY, In many cases lines of growth can be made out, and the shell may continue to increase in size for so long a time that a single valve of a Tridacna will be found to weigh a hundred and fifty-six pounds. In Nautilus the shell consists of a number of chambers (Fig. 126), each of which is larger than that which precedes it, and is formed by the animal as it increases in size; each of these chambers is separated. Fig. 126.—A Section of the Shell of the Pearly Nautilus, showing the successive Chambers occupied by the Animal. a, Mantle; b, dorsal fold; g, muscle; ii, siphuncle; k, funnel; n, hood;/), ten- tacles; s, eye; x, septa ; z, last chamber. from one another by a septum (x), and the whole mass is spirally coiled on itself. The chambers are connected with one another by a tube or sipliuiicle (ii), the presence of which has given rise to the belief that the whole series forms a kind of float by means of which the animal is enabled to remain at will on the surface of the water; such definite observations, however, as have been made on living specimens, and the fact that though (like the shells of Spirula) the shells are common enough, but the animals very rare, lead us rather to believe that Nautilus is essen- tially a dweller at the bottom of the ocean, Here,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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