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Page:Popular Science Monthly Volume 19.djvu/459

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eight days; the average being seven days. And this is a very interesting fact when we bear in mind the conclusion to which the inquiries of the Dutch meteorologists, and, more lately, those of the Scottish Meteorological Society, appear to tend, namely, that the shoals prefer water of about 55°. At 50° Fahr., the period of incubation is lengthened to eleven days; at 46° to fifteen days; and at 38° it lasts forty days. As the Forth is usually tolerably cool in the month of March, it is probable that Professor Allman's estimate comes very near the truth for the particular case which he investigated.

The young, when they emerge from the egg, are from one fifth to one third of an inch in length, and so extremely unlike the adult herring that they may properly be termed larvæ. They have enormous eyes, and an exceedingly slender body, with a yelk-bag protruding from its fore-part. The skeleton is in a very rudimentary condition; there are no ventral fins; and, instead of separate dorsal, caudal, and anal fins, there is one continuous fin, extending from the head along the back, round the tail and then forward to the yelk-bag. The intestine is a simple tube, ciliated internally; there is no air-bladder, and no branchiæ are yet developed. The heart is a mere contractile vessel, and the blood is a clear fluid without corpuscles. At first the larvae do not feed, but merely grow at the expense of the yelk, which gradually diminishes.

Within three or four days after hatching, the length has increased by about half the original dimensions, the yelk has disappeared, the cartilaginous skeleton appears, and the heart becomes divided into its chambers; but the young fish attains nearly double its first length before blood-corpuscles are visible.

By the time the larva is two thirds of an inch long (a length which it attains one month after hatching), the primitive median fin is separated into dorsal, caudal, and anal divisions, but the ventral fins have not appeared. About this period the young animal begins to feed on small Crustacea; and it grows so rapidly that, at two months, it is one and a quarter inch long, and, at three months, has attained a length of about two inches.

Nearly up to this stage, the elongated, scaleless little fish retains its larval proportions; but, in the latter part of the third month, the body rapidly deepens, the scales begins to appear, and the larva passes into the "imago" state—that is, assumes the forms and proportions of the adult, though it is not more than two inches long. After this, it goes on growing at the same rate (eleven millimetres, or nearly half an inch) per month, so that, at six months old, it is as large as a moderate-sized sprat.

The well-known "whitebait" of the Thames consists, so far as I have seen, almost exclusively of herrings, under six months old, and as the average size of whitebait increases, from March and April onward, until they become suspiciously like sprats in the late summer, it may