I50 



NATURE 



{Dec, 19, 187S 



un symmetrical 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 t 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 extra- 

 ordinary 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 accom- 

 panied 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 com- 

 mences, in eight species observed by Agassiz, very early 

 in life, while all the face-bones of the skull are quite car- 

 tilaginous, 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 httle 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 New- 

 port. 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 dorsal 

 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 eren 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 to- 

 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 inter- 

 fered with, and the direct and very active circulatioa 

 taking place to and from the heart and the orbital cavity, 

 constituting almost an ocular heart. 



The causes usually assigned for the development of 

 fishes with a binocular side are all more or less unsatis-^ 

 factory. It is known that in experiments thereon similar 

 conditions constantly fail to produce similar results. Of 

 the causes assigned the chief are : that the great width of 

 the body in flounders makes the resting on the one side 

 the most natural position ; but there are many fishes of 

 far greater width which swim vertically. The absence of 

 a swimming bladder has also been assigned as a good 

 reason, but some flounders have a swim bladder. Alex- 

 ander Agassiz hints that the true cause may perhaps be 

 that some broad fish may find it much easier to pursue their 

 prey while swimming close to the bottom. They are pro- 

 tected from detection by their coloured side resembling 

 sand, mud, and gravel. This would gradually lead to the 

 exclusive use of one side (should the fish lie on either 

 side) and would result^ in the atrophy of the eye, unless 

 the fish were able to transfer his eye to the other side and 

 so retain it. But then it will be asked, why do we not 

 find flat fish among the broad forms of every family of 

 fishes ? and, remembering that flounders are only found in- 

 the most recent geological deposits, why were they not 

 as common in earlier times as at the present day ? and, 

 above all, why was the tendency of the eye to change 

 transmitted from generation to generation and not the 

 binocular state itself? 



May not, suggests Agassiz, Giards' idea come to our 

 help here. Giards hints that the fundamental cause of 

 all asymmetry in the animal kingdom is due to a difference 

 in the strength of the organs of sense, and he gives in 

 support of this idea some most ingeniously explained 



