Page:Popular Science Monthly Volume 29.djvu/124

114. Some have taken to lying most frequently on one side and some on the other; but it is interesting to note that when a normally right sided individual has happened to lie with his left side uppermost that side becomes colored and distorted exactly the same as in his more correct brethren. This shows how purely acquired the whole habit must be. It points back clearly to the days when flat-fish were still merely a sort of cod, and suggests that their transformation into the un symmetrical condition is merely a matter of deliberate choice on their own part. Indeed, there seems good reason to believe that many young flat-fish never undergo this change at all, but swimming about freely in the open sea assume that peculiarly elongated and strange form known as the leptocephalic.

I don't mean to say that all leptocephali are originally the offspring of flat-fishes, but some probably are; and so a word or two about these monstrous oceanic idiots and imbeciles may not be here out of place.

Lolling about lazily in the open ocean a number of small, long, ribbon-like fish are frequently found, quite transparent and glassy in appearance, with no head at all to speak of, but furnished with a pair of big eyes close beside the tiny snout. They are languid, boneless, wormlike creatures, very gelatinous in substance, and looking much like pellucid eels without the skin on. For a long time these leptocephali (as they are called) were supposed to be a peculiar class of fishes, but they are now known to be young fry of various shore-haunting kinds, which have drifted out into the open ocean, and had their development permanently arrested for want of the natural environment. They are in fact fish idiots, and though they grow in size they never attain real maturity. If, as some authorities believe, many of these queer idiotic forms really represent stray flat-fish, then their symmetrical development once more points back to the happy days when the ancestral sole still swam upright, with one eye on each side of his head, instead of being distorted into a sort of aggravated squinter.

Besides the "reversed" specimens of soles and turbots—right-sided when they ought to be left-sided, and vice versa—occasional double or ambidextrous individuals occur, in which the dark color is equally developed on both sides of the body. Whether these impartial flat-fish are in the habit of turning over in their beds—whether they represent the uneasy sleepers of pleuronectid circles or otherwise—I am not in a position to state; but probably they are produced under circumstances where both sides have been frequently exposed to the action of light, which seems to have a sort of photographic effect upon the pigments of the fish's body. Everybody knows in fact that the upper side or back of most ordinary fish, exposed as it is to the sunlight, is darker than the lower side or belly; and this natural result of the solar rays has indirectly a protective effect, because when you look down into the water from above it appears dark, whereas when you look up from below the surface appears bright and shining; so that a fish is less