Popular Science Monthly/Volume 11/June 1877/Gar-Pikes, Old and Young II
|GAR-PIKES, OLD AND YOUNG.|
OF CORNELL UNIVERSITY.
THE writer's opportunities for observing the motions of the adult gar were too brief to enable him to describe them accurately. It is to be hoped that this fish may soon be placed in some public aquarium. But the motions of several young gars were carefully watched daily during three weeks.
The movements of the little gars, even the smallest, were very unlike those of the common little fishes, minnows or catfishes, which were placed with them. These latter seemed agitated, and splashed about in an indeterminate way. But the little gars, though they went like arrows when disturbed, usually remained almost at rest, or moved slowly about with a dignified, almost solemn air, as if conscious of very ancient and honorable lineage. They also have, as was remarked by Prof. Agassiz, the power of moving the head upon the neck; and occasionally the whole body was thrown into two or three undulations, resembling those of a short serpent; and so impressive is the air of supercilious self-possession that one might almost imagine them shrugging their shoulders at other creatures, including the bipeds of recent creation, who study their movements.
To sounds in general they paid no attention. But a tap upon the side of the vessel usually caused them to start and open the mouth, sometimes two or three times in succession.
It has already been said that the little gar first taken was recognized as such; yet the resemblance to the adult was mostly in the general elongated form of the body, and in several other respects there were marked differences. First, in color. The old gar is a bluish ash-color, or light gray; darker above, and lighter below, but with no distinct patches. All of the young gars presented a distinct though irregular dark stripe along the side of the body and head, crossing the eye. The belly, too, was almost white, and strongly contrasted with the darker regions.
Second, the smallest ones had no scales at all; but with one, 108 millimetres (about four and a quarter inches) long, the hinder half of the body showed outlines of the scales in process of formation, and the larger ones had the armor more or less fully developed. At about the same time the upper and lower borders of the tail become protected by several pair of pointed plates, the fulcra.
The third and most striking peculiarity of the young gars consisted in the existence of two tails, an upper and a lower. These are shown in Fig. 8, B.
The formation of these two tails, and their significance, will be considered further on; for the present, we are concerned with their Structure, their relative position, and their uses. The lower tail was
A, from a specimen twenty-two millimetres or seven-eighths inch long, enlarged four diameters. The ventral fin (Ve) is just appearing. The median fin is being absorbed between the fourspots referred to in Fig. 9. The lip of the tail is inclined upward, and the infra-caudal lobe is larger. In B the primordial fin has almost disappeared; the dorsal (D) and the anal (A) fins are quite large. The infra-caudal lobe is nearly as long as the tip of the original tail, which has been reduced to a slender vibratile filament. This specimen is forty-four millimetres or one and three-fourths inch long, and the tail is enlarged two diameters. C shows the tail of a specimen three hundred millimetres or nearly twelve inches long, of natural size. The filament is still further reduced, and the rays of the infra-caudal lobe form the end of the tail In D the fail is that of an adult, one-half natural diameter. The filament, the original end of the body, has wholly disappeared, and the infra-caudal lobe forms the tail. But dissection shows the spinal axis extending along the dorsal border to a point corresponding with the previous attachment of the filament. (Further description and discussion of these changes, with references to authors, may be found in a paper by the author, entitled "Notes on the North American Ganoids." "Proceedings of the American Association for the Advancement of Science," 1875, pp. 151-193.
evidently the caudal fin. It had several rays, and a rounded hinder border. But it was smaller in proportion than in the adult gar, and the middle rays were directed obliquely downward, instead of horizontally backward.
The upper tail is best described as a single fleshy filament, flattened from side to side, and tapering to a fine extremity. In the smallest gars it was longer than the fin below, in the older it was shorter, while in the adults no trace of it appears.
These two tails have very different movements. The lower, corresponding to the caudal fin of the ordinary fish, is used in three ways. When the little gar is in a gentle current, and wishes not to be carried downward, the fin is made to execute a series of undulatory movements, such as have been described by Prof. Agassiz respecting the dorsal fins of young pipe-fishes, etc., and such as the writer has observed with the long dorsal fin of Amia.
This tail is also strongly flexed to one side, as with ordinary fishes, in order to change the course. And it is rapidly moved from side to side for all sudden and rapid locomotion, as when frightened.
The movements of the filament were first described by Prof. Agassiz, and he called attention to them upon several occasions. But his descriptions are very brief, and, upon one point, seem to require modification.
The filament is in almost constant vibration. Occasionally, when the gar is at rest, and perhaps also when it is turning, or rapidly swimming, the filament is not used. But usually the vibrations are so rapid that the tip of the filament is invisible, excepting as an indistinct blur. Generally, it is directed backward and slightly upward, but at times it is bent to one side, or elevated to nearly a right angle with the body, the tip all the while in constant vibration. Those who have watched the tail of an irritated rattlesnake, or even of a common striped snake, under strong excitement, may form a pretty correct idea of the nature of this movement. It was characterized by Prof. Agassiz as "involuntary;" and so it may be regarded, since its rapidity is such as to preclude the idea of a separate volition for each movement. But the gar, evidently, has entire control of the vibrations; for they are more or less rapid at different times, and are occasionally intermitted; the position of the whole filament is changed at will; finally, the muscular bands upon each side of the cartilaginous rod, which runs through the filament, consist of the striped variety of muscular fibre, as are the other voluntary muscles.
This is all the writer has seen of living young gar-pikes. But the explanation of the peculiar double tail is furnished by some still younger specimens, the smallest of which is shown, enlarged, in Fig. 9.
These little gars were scooped out of the Red River, near Shreveport, Louisiana, in the spring of 1871, by a lad only ten years old, who had heard the writer say that he wished for very small fishes. At that point these young gars were then as abundant as minnows, as easy to catch, and commercially as worthless. All of them are less than two inches long, and among them are two about three-fourths of an inch in length. These last are not only much smaller than any previously examined by naturalists (so far as known to the writer), but they also furnish the clew to the double tail, and suggest some important paleontological considerations.
While earnestly expressing his appreciation of the value of these little gars, the writer finds himself compelled to exemplify the proverbially ungrateful and dissatisfied nature of zoölogists by regretting that there were not more of them, and that some were not very much smaller, or even still within the egg.
In this connection one is reminded that now, as a rule, the smallest rather than the largest are desired by naturalists. The giants are curiosities, and interesting as showing the capacity for growth; but the mysteries of development, the relations of apparently diverse forms, and the order of geological succession, are best revealed by the apparently most insignificant.
A good illustration of this inverse ratio between size and value is contained in the following passage from Prof, and Mrs. Agassiz's "Journey in Brazil:"
Of the two smallest gars, one is nearly colorless, while the other is marked very much as are the older ones. They are 18 millimetres (a little less than three-fourths of an inch) in length. The head is short and flattened, with slight indications of teeth on the edges of the jaws. With one of them the ventral fins have not appeared; with the other they are represented by minute white elevations. Each pectoral consists of a fleshy lobe, surrounded by a thin fringe or border.
The hinder end of the body tapers to a point, as with Amphioxus, the extremity being slightly bent downward. At the junction of the middle with the hinder third of the body commences a delicate median fin, colorless, and without rays for the most part, and extending around the tip of the tail forward to the vent, thence forward to about the middle of the body. Such a primordial median fin exists in the young of all fishes whose development has been studied. The permanent fins seem to result from the formation of cartilaginous or bony rays either throughout most or the whole of its length, as with lampreys and common eels; or at several points, as with the mackerel and Polypterus; or at three, or two, or only one, as with the cod, the blue-fish, and the pickerel. The intervening portions disappear. The hinder part of the primordial fin of the smallest gars presents four points of darker coloration, two above and two below. The posterior upper spot presents no rays, and later seems to disappear. The other three are evidently the beginnings of fins. The anterior above and that below occupy the positions of the future dorsal and anal fins. The destiny of the hinder lower spot is better seen by comparison with larger examples.
The series given in Figs. 8 and 9 shows that, as the gar increases in length, the primordial fin disappears, the dorsal and anal increase, and the end of the tail becomes more slender and pointed. But the most striking change consists in the enlargement of the hinder lower spot into what may be called the infra-caudal lobe. The rays of this become longer and more numerous. They project beyond the margin of the primordial fin, so as to leave a decided notch, as in Fig. 8, A.
In Fig. 8, B, the end of the body merits the name of filament, and the relative size of it and the lobe is reversed. Afterward, partly by more rapid increase of the lobe, and partly by absorption of the filament, the latter seems smaller and smaller, and at last disappears; so that the lobe, from having been at first an outgrowth from the filament, finally becomes the whole of the tail or caudal fin.
It appears, then, that the hinder end of the body undergoes considerable change before reaching the adult condition. Aside from the partial disappearance of the primordial median fin and the gradual development of the ventrals, the dorsals, and the anal, the caudal fin assumes at least three distinct forms. The first is lance-shaped and simple, like that of Amphioxus, the eel, the lamprey, salamanders, and tadpoles. The second is compound, with a slender filament above and a broader fin below, as with some sturgeons and sharks. The third consists entirely of the lower fin, which is enlarged and brought into a direct line with the body, the longest rays being a little above the middle. Its upper and lower borders are now thicker and stronger than the intermediate portions; whereas in the first stage the cartilage and muscle are in the centre, the upper and lower borders being very thin.
In short, the tail of the gar-pike undergoes a decided transformation. And one naturally inquires, "What is the occasion for it?"
It is so recently that all structural differences and changes were supposed to be readily explicable upon the doctrine of final causes, that we naturally turn first in that direction. Some transformations certainly seem to relate very distinctly to the welfare of the individual, as when the caterpillar becomes a butterfly, and when the aquatic larvas of mosquito and dragon-fly change their forms with their habits and modes of life. So, among the vertebrates, it is obvious that the tadpole is by no means adapted to the necessities of the frog and the toad; and the intermediate stages, resulting from the gradual loss of the tail and the acquisition of legs, while perhaps not particularly suited to either aquatic or terrestrial locomotion, seem to be required in order to permit the development of the lungs and the accompanying disappearance of the gills.
But can the transformations of the gar-pike's tail be thus accounted for? According to present knowledge and justifiable inference, the Lepidosteus not only passes the whole of its life in the water, but is also, from first to last, an active, predaceous fish, requiring all possible advantages of form and fin in order to overtake its prey.
Since no marked change occurs in the general form of the body, we may perhaps assume that it is perfectly well adapted to the fish's needs; although this suggests the general inquiry as to the cui bono of the almost infinite variations from the ideal form supposed to be best suited to aquatic locomotion.
But do we know, or can we easily infer, any differences in the necessities or the manner of life of the Lepidosteus at different ages, which may account for its having a tail first like a lamprey's, then like a sturgeon's, and, finally, like that of Amia?
It may be suggested that the rapid and, at most, invisible vibrations of the filament enable the young gar to glide stealthily upon its prey. But the very young would seem to be even more in need of such precaution, and with them the tail is relatively as large as in the adult, although differently shaped. Finally, even if we conclude that the three distinct stages of the tail are perfectly adapted to certain hypothetically unlike necessities, what shall be said of the intermediate conditions? While growing, the infra-caudal lobe must be rather a hinderance than a help to the movements of the primitive tail; and while disappearing, the filament, being useless, must be, if anything, an incumbrance.
Shall we, then, conclude that these changes in the appearance of a single individual are for the sake of variety—as some would explain the great diversity of specific form and coloration among animals and plants?
At the present day, neither of the explanations above given is likely to wholly satisfy the large class of thinkers who, whether or not they accept any particular evolution doctrine, are inclined to believe that there is, in many cases, a more or less exact parallelism between the changes which occur in the development of an individual, the successive forms of geological times, and the series of living forms, lower and higher, or more generalized and more specialized.
In the smallest gar here described, and presumably in still younger examples, the axis of the body, represented by the notochord or primitive vertebral column, is nearly horizontal, about midway between the upper and the lower borders of the tail. This is likewise the case with the lowest known vertebrate, Amphioxus; with the forms next above, the hag-fishes (Myxine and Bdellostoma) and lamprey-eels (Petromyzon); with the larvæ (tadpoles) of frogs and toads; and with the adults of the aquatic and tadpole-like salamanders, Menopoma and Menobranchus.
Finally, such a tail exists in the Dipnoans, or mud-fishes, of Africa, South America, and Australia (Frotopterus, Lepidosiren, and Ceratodus), which have some striking affinities with Batrachians, but are usually regarded as fishes, and are, perhaps, the best illustration of generalized forms.
To this variety of tail, Cope has applied the name isocercal; Huxley calls it diphycercal, and gives as an example Polyhyterus, where, however (as in Calamoichthys), the "end of the notochord is hardly at all bent up." Wyman, finding this kind of tail in the embryo of a skate, called it protocercal, and, on some accounts, this seems the more suitable name.
As the gars grow older, the relative length of the filament and the infra-caudal lobe constantly changes. At first the former is the longer; in a specimen 108 millimetres long, their tips coincide; in one 142 millimetres long, the lobe projects beyond the filament; and in a third, 300 millimetres long, the filament is much the shorter, is ragged and attenuated, and during life was rarely employed. This second stage, or rather series of stages, has several counterparts among living Selachians and Ganoids. The most accurate resemblance is presented by the shovel-nosed sturgeon of the Mississippi River (Scaphyrhynchus). The filament is excessively elongated in Chimera, and exaggerated as to both length and breadth in the thrashing-shark (Alopias). But, with many sharks, the common sturgeons, and the spoonbill (Polyodon), the size of the infra-caudal lobe is so nearly that of the filament as to give the whole tail a nearly symmetrical outline, and lead zoologists to speak of the "upper lobe," whereas it is really the bent-up end of the body. This kind of tail is called heterocercal.
The gars above mentioned are supposed to be the young of the Lepidosteus osseus. Just at what size the filament wholly disappears in that species is not known. But with the smaller and proportionally shorter species, L. platystomus, there is no sign of the filament when eighteen inches in length. The tail might then be thought, at first sight, to be symmetrical. But the longest rays are a little above the middle, and dissection shows that the spinal axis is continued backward and upward as a cartilaginous rod, terminating at the upper border, just under the hinder pair of fulcra, and at the point where the filament was attached. The rays are all attached to the lower border of the spine; and there is only a lower lobe of the tail.
A similar structure exists in the tail of Amia, which Prof. Huxley gives as an example of heterocercal tail. It seems better, however, to discriminate between it and the previous stage, where the upper lobe (filament) exists, and it may, therefore, provisionally be called the masked heterocercal, or perhaps the pseudo-homocercal.
Prof. Huxley has more recently given figures and descriptions of the tail of embryo Teleosts (Gasterosteus), in which the structure is nearly identical with that of the adult Amia and Lepidosteus.
Fig. 10.—Diagrams intended to illustrate the Correspondence of the Successive Stages of Transformation of the Tail of Lepidosteus, with the Tails of Certain Living Forms more and less generalized, and of Certain Fossils more and less ancient.
A, the first or protoceral stage, where the end of the vertebral column (Vc) is horizontal and divides the tail into upper and lower lobes nearly equal in size. B and C, the heterocercal stage, where the original tail is more or less elevated by the lower or infra-caudal lobe (IC), and becomes the filament (Fi), usually called the "upper lobe." In D the infracaudal lobe is longer than the filament, and in E the latter has wholly disappeared, and the tail assumes the last or "masked heterocercal" condition.
The same author concludes that in many adult Teleosts the posterior end of the spine is more or less strongly bent up, although the tail is outwardly nearly or quite symmetrical.
But when, as in the majority of species, the hinder border is emarginate, so as to form an upper and a lower lobe, the former is never known to contain any extension of the spine; although some South American Goniodonts have the upper ray prolonged into a sort of filament, yet in other forms the lower ray is similarly elongated, and neither can be compared with the true filament of the young gar or the upper lobe of sturgeons and sharks.
It may not be possible to draw a sharp line between the tail of most adult Teleosts, and that of Amia and Lepidosteus, but perhaps the old term homocercal can be employed for the former.
Upon the whole, it would appear that the tail of the youngest Lepidosteus is protocercal like those of the lowest vertebrates and the generalized forms called Dipnoans; that the second or obviously heterocercal stage is comparable with the tails of sharks and sturgeons, while the last stage seems to correspond quite closely with that of the teleostean embryo. And, as the Teleosts are almost universally regarded as the most specialized group of fishes, there appears to be a pretty close agreement between the successive stages of Lepidosteus and the rank of the forms or groups with which comparison has here been made.
The corresponding geological series is less complete and satisfactory. No forms resembling Amphioxus or the hag-fishes and lampreys have yet been found fossil, although all, excepting the former, have horny teeth, of which, it would seem, some traces might well be preserved.
But among the oldest fishes are some described by Huxley whose tails are apparently protocercal. The resemblance to the earliest stage of Lepidosteus is emphasized also by the existence of two dorsals and two anals.
Fossil species of Amia and Lepidosteus have recently been discovered by Prof. Marsh in the Tertiaries of Western America. The Megalurus of the European rocks had a tail strongly resembling that of Amia, but this kind of tail is not known among the palæozoic rocks, and Teleosts are first found in the Cretaceous, becoming more and more numerous up to the present time.
But among the earliest known fossil fishes are some in which the end of the spine is not at all bent up; the tail is protocercal. And, with two genera (Glyptolæmus and Gyroptychius) described by Prof. Huxley, it may be possible to determine the correspondence between the two dorsals and anals and the two pair of differentiated spots upon the primordial median fin of the youngest Lepidosteus.
So far as the writer is able to ascertain, the protocercal tail is less frequent in later geological epochs, while the obviously heterocercal form, as with Palæoniscus, etc., becomes more and more abundant.
Apparently, therefore, the order of succession of the three or four kinds of tails coincides, in the main, with the series seen in the growing Lepidosteus; and the geological, the zoölogical, and the embryological series, upon the whole, have a recognizable correspondence.
So far, the writer has endeavored to give an outline of the natural history of the gar-pike as a peculiar American fish, concerning which little has hitherto been published even in strictly scientific works, and almost nothing in a form generally accessible.
In so doing he has purposely avoided the presentation of controversial points, or, in reference to the nomenclature of the air-bladder and of the tail, has presented opposing views, with an abstract of the evidence, so far as known to him; admitting his inability, as yet, to form a definite conclusion.
But there is another and, in some respects, most interesting and important light in which the gar-pike may be considered, namely, as to its relations with other fish-like forms.
Is Lepidosteus merely a somewhat peculiar fish? Or may it, with Polypterus and some fossils, be separated as a distinct group? Or should there be added to this group Amia and the sturgeons? Or should the catfishes and their kindred, with the pipe-fishes, globefishes, and others, be likewise included?
Upon what grounds may this group be defined? What is its grade, class, sub-class, or order? And how may it be subdivided?
Attempts have been made to find answers to these questions by the study of the scales, the skeleton, the limbs, the gills, and various internal organs. The embryology of the sturgeons is not fully known, while nothing whatever has been observed of the earlier stages of the so-called typical Ganoids.
It is probably within the truth to say that, from the time of Cuvier down, no two authors upon fishes agree upon all the points, while any contemporary discussion, whether verbal or in print, is almost certain to be attended with a degree of heat quite incompatible with the apparent importance of the subject.
The fact is, however, that the so-called Ganoids occupy a very peculiar position. None of them can be touched without affecting the entire series of fish-like forms. Ichthyology is in a state of instability, and every important new fact, every decided expression of opinion by high authority respecting the Ganoids is liable to require a revision of all our ideas.
To present even an outline of the many views, and of the facts and considerations upon which they are based, would require an entire article, with many figures and some anatomical description.
To the reader who has become interested by the foregoing imperfect sketch of the gar-pike, and who has the good fortune to live within reach of it, of Amia, and of the sturgeons, the writer would earnestly recommend a careful and systematic investigation of their habits and their structure—especially that of the brain—and of their development, as likely to furnish the most reliable basis for their classification.
- It may have a morphological significance, as suggested further on respecting the fossil Glyptolæmus.
- This rod consists of the notochord, and a slender prolongation of the spinal cord, surrounded by a cartilaginous sheath.
- The writer has found the same condition in newly-hatched catfishes (Amiurus), and it has been observed in the embryo of a species of Cottus, by Mr. S. H. Gage, a student of natural history at Cornell University.