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THE HIPPOCAMPUS AND ITS YOUNG.

SOME VERY INTERESTING OBSERVATIONS respecting the young of the hippocampus, and their development in the pouch of the male parent have lately been given in the American Naturalist by the Rev. S. Lockwood. The species on which these observations were made was the Hippocampus Hudsonius, the common seahorse of the Atlantic coast of the United States, a species closely resembling the Hippocampus ramulosus of Europe, figured and described in The Field of Oct. 19, 1867.

In the seahorses the male only is furnished with an abdominal pouch, in which the eggs or spawn of the female are deposited, and in which they are hatched and the young retained some time after their exit from the egg, When captured the male fish is found to be exceedingly apt to expel the young from the pouch, even when in an immature condition. By very careful management, however, Mr Lockwood succeeded in so far satisfying the requirements of a “gravid” fish as to induce him to retain the young until they were fully developed. What follows we must allow Mr Lockwood to describe in his own words:

To-day near noon I observed three young sea-horses swimming about They had just made their début. Very minute creatures they were; but to my great joy, nearly perfect. From that hour the Pater-mater kept busy setting his progeny adrift. At the bottom of the vessel was a broken shell, put there for the attachment of the animal's tail, when fatigued by swimming—as the seahorse is very easily tired—and this, monkey-like, is its favourite mode of taking rest. The shell afforded real help in the labour of extruding the young, which is in no sense a parturient process, but, on the contrary, is entirely mechanical, and in the present case was effected in the following manner: With its abdomen turned towards the shell, its tail attached to the under part of it, the body erected to its full height, the animal, by a contractile exertion of the proper muscles, would draw itself downwards, and against the shell thus rubbing the pouch upward, and in this simple yet effective way expelled the fry at the opening on top of the sack (see Fig. 1). It was said above that the seahorse is soon wearied, with even moderate exertion; hence, probably, it was that these repeated acts were each followed by a few minutes of rest. Indeed, the extrusion of its young lasted for nearly six hours, from three to six individuals being set free at a time.

[Illustration] Fig. 1.

The scene that followed was one of singular and lively interest. I was nervous with delight, and wished that every naturalist could see it for himself.

Using my best judgment—for, owing to the mazy motion of this tiny throng, counting was out of the question—I set the number down as not far from a thousand. Each measured from five to six lines in length. Very minute creatures, truly, when one considers how large a portion is taken up by the tail, which organ was of but little more than thread-like dimensions. We might suppose it would require a few days for the young hippo to find out the remarkable monkey-like endowment of its tail. Not so. Only look at what my own eyes beheld many a time, when a “stampede” of these little colts was going on, although they were but one day old. There come two little hippos, each swimming in a direction at right angles to that of the other. Just at the point of passing, one, lasso-like whips his caudal extremity round that of his fellow, who, of course, in like manner, returns the caudal compliment, which, to speak technically, acts as a “double lock.” Of course both pull, and, by a natural law, the force is exerted in exactly opposite directions, and the right angle is resolved into a straight line. It is but poor head-way they make, nor does it mend the matter much that a third little fellow comes giddily on, and, switching his tail, takes a hitch at that precise point in space where the other two met. Now a triple force is exerted, and the effect is, with two straight lines to project three obtuse angles. And so the three toil on, obtusely labouring in statu quo. But a droller sight is that of yonder juvenile Lophobranch, who seems to be of somewhat belligerent proclivities, as he is leading by the nose a weaker member of his own species, having with his caudal extremity noosed him on the snout.

Unfortunately these little fish perished in a few days, none living beyond a fortnight.

Mr Lockwood maintains that the function of the pouch of the male hippocampus is not merely the reception of the young, but that they are also nourished whilst they are retained within it. He states that at the time of receiving the ova the walls of the pouch are thick and firm with fat, but that on the expulsion of the developed young they have become thin, flaccid, and destitute of adipose matter which has been used for the support of the young during their existence in the pouch. In the first instance the newly hatched fish are in a very rudimentary condition, and bear but a remote resemblance to the mature animals. Figure 2 represents the young that were prematurely expelled at a period which Mr [Illustration] Fig. 2. Fig. 3. Lockwood regarded as twelve days after the commencement of the incubation. Figure 3 shows one of these young fish magnified. It may be observed that the yolk sac is not yet absorbed, that the muzzle is short and wide, differing greatly from the prolonged jaw of the mature animal. The tail is round, instead of being quadrangular, and is so small that the dorsal fin appears to be much further back than in the adult.