About Fishes' Heads

Abstract

IN a former number (vol. xvii., p. 286), in a note “About Fishes’ Tails,”we called attention to some recent observations of Alexander Agassiz on the young stages of some fishes, in which he showed the wonderful changes that, as development went on, took place in their caudal fins; yet strange though these changes are, they seem as nothing to those that take place in some fishes’ heads, and the facts first noticed by Steenstrup, and the theory which, by a marvellous power of intuition, he built up thereon, as to the eye in a flounder passing from the right side of its head to its left, have been in a great measure confirmed, and perhaps in a greater measure added to, by the painstaking observations quite recently published, of Alexander Agassiz,1 from which it would now seem very certain that even the most shapeless adult fishes begin their life as quite symmetrical young creatures. No more unsymmetrical fish can probably be found than an adult flounder with its unsymmetrical tail, with its twisted head, with its two goggly eyes—brought together on the one side of that head—and yet examine a flounder while yet young. “The one I captured,” writes Agassiz, “was so transparent as to rival the most watery of jelly fishes. When placed on a flat glass dish it could only be distinguished by allowing the light to strike it in certain directions, otherwise all that was visible were the two apparently disembodied bright emerald eyes moving more or less actively. It was over an inch in length, the position of the eyes was perfectly symmetrical, and they were placed at a considerable distance from the anterior extremity of the snout; the dorsal fin extended almost to the nostrils.”From this beautiful symmetry how then did the strange want of it in the adult fish arise? Long ago (1863) this question presented itself to Steenstrup. He had a small number of very young flounders preserved in alcohol, and from an examination of these he answered the query thus:— The young flounder, after a short time, takes to lying on its right side, why no one can tell, but with this result that the eye of that side begins to turn inwards, and passing through the tissues of the head, transfers itself to the left side. So strange seemed this explanation, that Malm's observations, in which he seemed to show that this apparent transference was really due to a torsion or twisting of the entire head, appeared to some to be, perhaps, the most probable explanation of the extraordinary phenomenon described by Steenstrup, and yet in Steenstrup's paper he very clearly showed that any ordinary torsion of the head of a flounder on its axis was wholly insufficient to explain the final position of its eyes. Since 1863 a good deal has been written upon the subject of the want of symmetry in the heads of the so-called flat fishes, more especially by Sir Wyville Thomson, Dr. Ramsay Traquair, Dr. Schiodte, Dr. Klein, Professors Reichert and Canestrini; but the most important and the latest memoir is the one just published by A. Agassiz, which forms a second part of his memoir, “On the Young Stages of Osseous Fishes,”and is devoted chiefly to the development of the flounders. This memoir is accompanied by eight excellent plates, some of which show very well the changes of form through which some of the young flounders pass. The young flounders of some species attain a considerable size ere they show the least tendency to favour one side more than another, and before there is any change in the position of the eyes. They then swim vertically, at least when they come up to the surface to feed. This they will do on bright sunny days, about ten o'clock in the morning, while the water is very smooth, and they will then be seen to devour greedily swarms of embryo crustaceans of all orders. Some will after a while settle down on their left sides, which then in time become colourless and blind, these would be called dextral, while in some just the reverse takes place; but no matter on which side they take to resting on, the exchange is the same. First there is a slight advance of the eye of the blind side towards the snout, then this rises higher and higher towards the medial line of the head; it now becomes more and more visible from the coloured side, until at last it quite passes over. This transfer commences, in eight species observed by Agassiz, very early in life, while all the face-bones of the skull are quite cartilaginous, and, by a combined process of rotation and translation, it is completed long before these have become ossified. So far these observations of A. Agassiz were completely in conformity with the observations of Malm, who, it will be remembered, did not trace the changes undergone during the process; and they seemed to be completely antagonistic to the idea of Steenstrup, that the eye from the blind side passed through the tissues of the head and came out on the coloured side. But in the late summer of 1875 a little shoal of some fifteen quite transparent flounders were captured by Agassiz, on a quiet and brilliant morning, on the surface-of the water at the mouth of the harbour of Newport. They were swimming vertically, and violently rushing after the minute entomostraca which swarmed on the surface. They were at once transferred to shallow glass jars, in which they would remain at the bottom on their right sides, for hours immovable. When disturbed they were rapid in their movements, frequently jumping out of the water. When swimming vertically they usually moved obliquely, the tail being carried lower than the head. When one of these was looked at in. profile, its right eye could be seen through the head, slightly in advance and a little above the left eye; owing to the great transparency of the body, the right eye was then nearly as useful as if placed on the left side. Gradually it rose, until in about six days it was well above the left eye; shortly after, wonderful to relate, it was-seen to sink into the tissues at the base of the dorsa fin between this and the frontal; slowly it sank until the huge orbit became reduced to a mere circular opening. Little by little this became smaller and smaller, the eye pushed its way deeper into the tissues, until an additional opening was formed on the left side. At this stage there were three orbital openings, though of course but two eyes The original or right-orbital opening soon became closed and the coloured side had its two eyes. Thus was the. suggestion of Steenstrup proved to be correct by careful observation on a living form, and what is of even greater interest, A. Agassiz is, from having thus, as it were, seen, all round the subject, enabled to suggest that the difference between these two methods of the transference of these eyes is not so great as would at first appear, the eye that sinks through the tissues, only taking a slightly shorter cut to arrive at its destination than the one that travels round the frontal bone. He is also able to hint at facts and suggest thoughts thereon, that seem to us ta be as full of interest as of novelty. Only a few of these can we allude to, such as the great length of the optic nerve, which allows slack to be taken during the transfer of the eye, and yet does not cause the sight to be interfered with, and the direct and very active circulation taking place to and from the heart and the orbital cavity, constituting almost an ocular heart.

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