Page:Popular Science Monthly Volume 46.djvu/839

Rh therefore subjected to great variations of pressure, not unlikely at times approaching half a ton to the square inch. At its migratory period the shad rises to the surface from the deep, and the sudden change of pressure thereby occasioned would do great violence to its bodily structure but for an apparent special provision of Nature, to be later explained.

In the marine profound there are fishes in abundant variety that exist at depths of several miles, and perhaps also in its nethermost and as yet unknown deeps. Many of these fish seem to be limited to certain oceanic strata to which their organization may be specially adapted and to no other. From time immemorial finny creatures of soft and fluffy substance and distorted shape have been found floating, sometimes dead, sometimes barely living, upon the surface of the sea; but, until the explorations of the Challenger afforded the solution, their nature and origin remained a mystery. The fish then drawn up from the far deep presented upon emergence the same bloated and unsubstantial appearance, which, by the scientists of that famous expedition, was ascribed to the expansion of the contained gases incident to the sudden release of enormous pressure. This expansion had so ruptured or puffed out the tissues of the creature that its entire semblance was without doubt radically altered, the puffy and loosely coherent mass of flesh having originally been a firm and compact substance. Fish of such localized habitat, if venturing too far above their proper stratum, would, despite their utmost effort, be buoyed to the surface by the constantly enlarging volume of the imprisoned gases, dilating measurably with the diminution of the weight or depth of the overlying water, and to such mischances is due the occasional appearance of these forlorn castaways upon the wide bosom of the deep.

Against such injury or inconvenience the shad, in common with its congeners, is seemingly secured by an anatomical peculiarity not as yet fully understood, but believed to be a distinctive adaptation of Nature. In the shad's head there appear a number of tubes presumed to exist for the introduction into the blood and system generally of a sufficiency of water, which fluid may be absorbed or extruded with increase or with diminution of pressure, and the tissues become thus adjusted to the varying strain. . This presumptive function does not, of course, admit of experimental observation, and in the absence of such demonstration the mode of operation and, indeed, even the purpose of the singular tubes must to some extent remain a subject of speculation.

Of all our marine food fish shad may be said to be the most popular; as an edible and as a delicacy it appears upon the table of both the poor and the rich, being equally esteemed by the epicurean as well as by the unpampered palate. The Chinese have