Popular Science Monthly/Volume 21/June 1882/The Eye-Like Organs of Fishes

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ONLY a few biological studies can count on so general an interest as those which concern the diversities in the sense-life of animals. We wonder at the stories of snails and mussels that have ears in their feet, or on their backs, or in the folds of their mantles, or which, like the Argus of mythology, have many eyes, or which have eyes on all their limbs; or of those creatures which, like some fishes, have organs of taste all over their skin; or of animals on which have been discovered nervous organs that do not seem to relate to any of our recognized sensorial functions, but rather point to some sixth sense, unknown to us. To this class belong the phenomena presented by a group of bony fishes, living for the most part at extreme sea-depths, classified in the three related families of the Scopelids, Sternoptychids, and Stomiatids, which have lately received attention from naturalists. They are generally small fishes, often only an inch or less in length, and have on either side of their belly a row of bright spots, extending from the snout to the tail, that might be said to look like a double row of pearl-buttons fastened upon their skin-coat. Sometimes a third row is found extending from the head to the anal fin; and frequently single spots, often of considerable size, are scattered over the head and gills and over the sides of the fish. Several ichthyologists—among them B. Rafinesque, of Palermo; Delle Chiaje, of Naples; Risso, of Nizza; and Cocco, of Messina—have had their attention drawn, since the first decade of the century, to specimens of those creatures that have occasionally been washed ashore in storms; and the more recent deep-sea investigations have made several allied forms known. The old ichthyologists apparently never examined the spots very carefully, but simply described them as silvery mottles or light points. Leuckart seems to have given them the first critical examination in 1864, in Chauliodus Sloani, Stomias boa, and Scopelus Humboldtii, and came to the opinion from it that they might possibly be regarded as supplementary eyes. Ussow, of Moscow, published a paper in 1879 on the structure of the so-called eye-like spots in Chauliodus, Stomias, Astronesthes, Gonostoma, and Maurolicus, in which he expressed the conclusion that the spots in the

PSM V21 D175 Argyropelecus hemigymnus.jpg

Fig. 1.Argyropelecus hemigymnus, twice the natural size.

three first-named genera were real organs of sight, but that the structure of those in the other genera was of a quite different nature, and really glandular. In the same year Leydig published a work on the Chauliodus Sloani, in which he admitted the similarity of the spots of that species to eyes, but was disposed to regard them as transitional organs rather than as real eyes, and referred to one of his observations as indicating that they might have been luminous in life. Leydig has more recently examined ten other species of the families Sternoptychidœ and Scopelidœ (from specimens preserved in spirit), and has considerably advanced the solution of the question of the office of these organs.

The organs in the Sternoprtychids and the Scopelids show essential differences in structure, and a third type has been noticed in some scopelids. Hence, Leydig has described three classes of organs, consisting—1. Of eye-like organs; 2. Of organs of a glass-pearly appearance; and, 3. Of luminous organs. These three forms can be easily distinguished with a glass. The organs of the first class resemble brownish sacks filled with a gray matter; those of the second class brown-bordered, plate-shaped depressions, the ground and edges of which are covered by a film with a metallic luster; and those of the third class, confined to the genus Scopelus, present themselves as larger spots of a silvery luster, or a grayish pearl-color.

The eye-like organs—which we have already spoken of as arranged in rows along both sides of the lower central line of the body—are also found on the head about the nose and eyes, on the lids and skin of the gills, and, in the genus Chauliodus, in groups of much smaller spots within the cavities of the mouth and gills. The number of the spots, which hardly ever exceeds a hundred in the other genera, rises in this genus to a thousand and more. Their outward appearance is not quite the same in the different parts of the body, but passes from the form of a round sack to that of a cylinder; and some spots are of the shape of a bell or an ampulla. In the genus Argyropelecus (Fig. 1) the Organs are grouped. They consist of an integument of brown

PSM V21 D176 Ichthyococcus ornatus.jpg

Fig. 2.Ichthyococcus ornatus, twice the natural size.

pigment, which is coagulated from the thick skin and forms a ring-fold, or gather, dividing the interior into a forward and hinder part. Within this integument is a film of a bright metallic luster, which either underlies the whole of it, or only forms a belt at the mouth, and consists of iridescent threads or spangles lying in the thick skin. The gray inner mass is divided into two sections, a larger hinder part filling the sack, and a smaller forward part. The hinder part is always spherical, the forward part cylindrical, and the two together form a connected whole. To both parts appertains a radial striation proceeding from a frame-work that is continued within from a membrane inclosing the gray mass. The longitudinal section of the hinder part of the organ superficially resembles the cross-section of an orange. We have to deal here, however, not with a few pervading radiations, but with a hollow cone of radiations meeting in the center, a certain number of which stream out over the spherical circumference of the sack, and fill the neck-part debouching without, so as to give the figure of a cone of rays sunk into the sphere. The network is, like that of the orange, filled with small cells, a part of them strongly refracting the light, which pass toward the common point of radiation of both divisions into an opaque granular substance. A nerve is always present in the neck-region of this organ, the fibers of which appear to be lost in the granular midst of the spherical section whose exact histological relations have not been ascertained. Externally, the whole organ is inclosed in a lymph-chamber.

The glass-pearly organs are also distributed over the sides of the belly, the head, the gill-flaps, and the skin of the gills, and the three on the skin of the gills are always longer than the others. They are of the shape of a round disk a little sunken, with a body having a metallic

PSM V21 D177 Eyelike organs of the argyropelecus hemigymnus and stomias anguilliformis.jpg
Fig. 3.—Eye-like Organs from the Border of the Belly of Argyropelscus hemigymnus; longitudonal section, greatly magnified. Fig. 4.—Longitudinal Section of the Eye of Stomias anguilliformis (after Dessow), with the parts designated thus; interior vitreous substance (x); lens:(l); retine (r); pigment layer (p); iris-like fold (ir), and optic nerve (n), greatly magnified.

luster and overlaid with a curved transparent integument. An outer brown film of pigment is always present, with a layer of closely joined, regular, hexagonal plates, and a latticed jelly-tissue of delicate, radiated cells that form a network, and are lifted up under a roof-like, spiral-shaped concretion (Fig. 5). The nerve-bundles are also present. Quite similar, but distinguished chiefly by their larger size, is the structure of the so-called luminous organs which are present in the Scopelus Rafinesquii and Scopelus metopoclampus as brightly glittering, well-detined spots above the nasal openings and under the eyes, and which in Scopelus Humooldtii and Scopelus Benoitii exhibit the form and appearance of depressed pearl-spots.

Concerning the nature of these organs, Leydig denies that any of them are glandular, although Ussow admits that this may be the case with some of the fishes. The hypothesis that they are organs of a sixth sense has received no confirmation. There remains, then, the theory proposed by Leuckart, Ussow, and Leydig, and accepted by Semper as undoubtedly correct, that they are real subsidiary eyes, like the eyes of mussels, etc. Leuckart and Ussow believed that they were able to distinguish a lens, a vitreous substance, and a retina, and the latter has published drawings of those parts; but the careful examinations of the structure of the organs and comparisons between it and the eyes of mollusks have led Professor Leydig to doubt this opinion; for he has observed that, when the fish swims horizontally, the mouths of the supposed eyes are turned, not toward the light, but downward, toward the dark bottom. Still less do the glass-pearly organs resemble eyes. Leydig is rather disposed to believe that he can with great probability recognize an identity in their structure with that of the electric and pseudo-electric organs of some fishes, particularly in the jelly-tissues and the disposition of the nerve-endings. According to this view, each of the disks would in itself correspond to a chest of the electric organs. The round shape of the disks may be explained by their isolated situation, there being no pressure of one upon another to make them angular. A similar diversity prevails in the form of the electric and pseudo-electric organs to that existing in the organs which we are considering, while the homology of the two is strikingly expressed in their similar situation and distribution. Leydig believes that two series of formations of this kind have been developed,

PSM V21 D178 Two glass pearly organs from the side of scopelus humboldtii.jpg

Fig. 5.—Two "Glass-pearly" Organs from the Side of Scopelus Humboldtii, moderately magnified.

one of which leads through the pseudo-electric organ of the Gymnarchus niloticus and the disk-like organs of the Scopelids to the real electric organs, while the other series includes the eye-like organs of the sternoptychids; an apparatus which is also represented in the larvæ of salamanders.

The appearance of this phenomenon in the amphibia, frequently observed as they approach the fish type, should point to some definite connection between the activity of those organs and water-life; but the nature of this activity, whether electricity is developed by it or not, is still veiled in complete darkness.

These organs have been regarded by many as luminous organs. A single glance shows that the body and lateral walls of the disk shine with a silvery and golden luster, but not different from that of the background of a fish's eye when viewed before a screen. More striking is the appearance in the case of the larger organs of the head in certain species, which are preeminently marked by it as a luminous apparatus. But, if the sole object of the apparatus were the collection and reflection of the light which fell upon the fish, its complicated structure in other respects, and its innervation, would be superfluous and still more incapable of explanation. We have, however, an observation that seems to show that these organs not only collect light, but are also really phosphorescent. The distinguished naturalist of the Challenger Expedition, Willemoes-Suhm, now deceased, saw Scopelius phosphorescent in the night, of which he says: "One of them hung in the net like a shining star as it came out of the darkness. Possibly the seat of the light is in the peculiar side organs, and it may be that this phosphorescence is the only source of light in the great depths of the sea." The thought that in the dark abysses of the deep sea every animal carries its lantern as the miner carries his lamp on his head, is a very fascinating one; and, indeed, Herr Willemoes-Suhm observed several other fishes that were provided on the smooth head and on the head-beard with "a remarkably large sense-organ." Valenciennes has also remarked of the genus Hemirarnphus that it bears a strongly glittering phosphorescent pustule on the tip of its tail. Although the majority of these animals have never been observed in a living condition, we might easily agree to the opinion that the organs

PSM V21 D179 Luminous organs in the ocular region of the scopelus rafinesquii.jpg

Fig. 6.Scopelus Rafinesquii, twice the natural size. Two luminous organs in the ocular region.

of all three categories serve as a more or less perfect illuminating apparatus; and, if we compare Professor Leydig's sections of them, this opinion, which is only very apparent at the first view, becomes extremely probable. Especially does the section of the eye-like organ of Argyropelecus and Ichthyococcus resemble the illuminating parts of a projection-apparatus. If we conceive the granular spot in the center, into which the nerves enter, as the source of light standing in the middle of the apparatus, there are likewise behind this the concave reflector, and in front of it the diaphragm through which the concentrated cone of rays is thrown outward under a strong refraction. In the pearl-like organs, also, if we have understood Professor Leydig's description aright, a curved, refracting body seems to lie on the side of the organ that is turned outward. We should thus, if our presumption is confirmed, have here not a simple illuminating organ, but a complete optical illuminating apparatus in different degrees of perfection, throwing out in an extremely concentrated condition, by means of a concave mirror and lenses, the phosphorescent light generated within it; and the fishes under consideration would be fully equipped with a series of little, button-shaped illuminating apparatuses.

I may assert here that there is nothing hazardous in this idea. As Professor Leydig has maintained, the "eye-like," and the" pearl-like," and the really luminous organs, are of thoroughly homologous structure, and we know of the latter, the only ones that have been observed in a living animal, that they emit a star-clear light. If, now, Nature has provided us with a most wonderful camera-obscura in our eyes,

PSM V21 D180 Caudal extrimity of scopelus humboldtii with glass pearly organs.jpg

Fig. 7.—Caudal Extremity of Scopelus Humboldtii, with "glass-pearly" organs, and a large pearl-spot.

why may she not also have produced a much simpler light-house lantern—provided, of course, that such an apparatus could be useful to the animal? I have already had something to say concerning the uses of their luminous apparatus to different animals ("Kosmos," vol. vii, p. 479), and have endeavored to show that their principal service is probably as a means of exciting fear. At any rate, the opinion may be given up that the light diffused by the deep-sea animals is a means of clearing up the purple darkness below, or, as some have thought, of producing the diversified hues of the deep-sea animals. Animals living in the dark do not require light for their existence, as is demonstrated by the numerous blind cave-animals. The opinion, also, that the luminous fish make their prey in any way visible by means of the organs subsidiary to their eyes could not in any degree help to account for the existence of luminous apparatus on the lower part of their bodies, for their eyes would not be able to see what those organs lighten up; but such organs might very well make the animal more visible from a distance, and might thereby serve a similar purpose with the protective colors of the animals of the upper world, especially if the appearance were associated with a disagreeable taste or smell. Only in some such manner as this can we account for the luminous organs, for example, of a crustacean that was brought up by the Challenger Expedition from a depth of nineteen hundred fathoms, and which was totally blind.

Professor Leydig remarks upon this point that the luminosity is, for the most part, only a subsidiary shining dependent on the secretion of a fatty body, and that the significance of the formations as electrical and pseudo-electrical organs is not altered on that account. We might also remark on this subject that, according to Kölliker's observations, the luminosity of many animals is under the influence of the will, so that the innervation of the phosphorescent organ no longer seems superfluous; and that, according to Jousset de Bellesme, glow-worms cease to shine as soon as their principal ganglion is removed. Moreover, according to Bellesme's observations, the glow may be produced by electrical as well as by nervous excitation. At all events, the hypothesis which I have submitted appears to me to be worthy of a searching examination.—Translated from Kosmos.