Popular Science Monthly/Volume 34/November 1888/Problematical Organs of Sense
By Sir JOHN LUBBOCK.
IN addition to the organs of which I have attempted in the preceding chapters to give some idea, and to those which from their structure we may suppose to perform analogous functions, there are others of considerable importance and complexity, which are evidently organs of some sense, but the use and purpose of which are still unknown.
"It is almost impossible," says Gegenbaur, "to say what is the physiological duty of a number of organs, which are clearly sensory, and are connected with the integument. These enlargements are generally formed by ciliated regions to which a nerve passes, and at which it often forms enlargements. It is doubtful what part of the surrounding medium acts on these organs, and we have to make a somewhat far-fetched analogy to be able to regard them as olfactory organs."
Among the structures of which the use is still quite uncertain are the muciferous canals of fishes. The skin of fishes, indeed, contains a whole series of organs of whose functions we know little. As regards the muciferous canal, Schultze has suggested that it is a sense-organ adapted to receive vibrations of the water with wave-lengths too great to be perceived as ordinary sounds. Beard also leans to this same view. However this may be, it is remarkably developed in many deep-sea fish.
In some cases peculiar eye-like bodies are developed in connection (though not exclusively so) with the muciferous canal. Leuckart, by whom they were discovered, at first considered them to be accessory eyes, but subsequent researches led him to modify this opinion, and to regard them as luminous organs. Ussow has more recently maintained that they are eyes, and Leydig considers them as organs which approach very nearly to true eyes ("welche wirklichen Sehorganen sehr nahe stehen"). Whatever doubt there may be whether they have any power of sight, there is no longer any question but that they are luminous, and they are especially developed in the fishes of the deep sea.
These are very peculiar. The abysses of the ocean are quite still, and black darkness reigns. The pressure of the water is also very great.
Hence the deep seas have a peculiar fauna of their own. Surface species could not generally bear the enormous pressure, and do not descend to any great depth. The true deep-sea forms are, however, as yet little known. They are but seldom seen, and when obtained are generally in a bad state of preservation. Their tissues seem to be unusually lax, and liable to destruction. Moreover, in every living organism, besides those usually present in the digestive organs, the blood and other fluids contain gases in solution. These, of course, expand when the pressure is diminished, and tend to rupture the tissues. The circumstances under which some deep-sea fish have occasionally been met with on the surface bears this out. They are generally found to have perished while endeavoring to swallow some prey not much smaller, or even in some cases larger, than themselves. What, then, has happened? During the struggle they were carried into an upper layer of water. Immediately the gases within them began to expand, and raised them higher; the process continued, and they were carried up more and more rapidly, until they reached the surface in a dying condition.
It is, however, but rarely that deep-sea fish are found thus floating on the surface, and our knowledge of them is mainly derived from the dredge, and especially from the specimens thus obtained during the voyage of the Challenger.
In other respects, moreover, their conditions of life in the ocean-depths are very peculiar. The light of the sun can not penetrate beyond about two hundred fathoms; deeper than this, complete darkness prevails. Hence in many species the eyes have more or less completely disappeared. In others, on the contrary, they are well developed, and these may be said to be a light to themselves. In some species there are a number of luminous organs arranged within the area of, and in relation to, the muciferous system; while in others they are variously situated. These luminous organs were first mentioned by Cocco. They have since been studied by Günther, Leuckart, Ussow, Leydig, and Emery. Lastly, they have been carefully described by Günther, Moseley, and von Lendenfeld, in the work on "Deep-sea Fishes" in vol. xxvii of the "Challenger Reports." The deep-sea fish are either silvery, pink, or in many cases black, sometimes relieved with scarlet, and, when the luminous organs flash out, must present a very remarkable appearance.
We have still much to learn as to the structure and functions of these organs, but there are cases in which their use can be surmised with some probability. The light is evidently under the will of the fish. It is easy to imagine a Photichthys (Fig. 1) swimming
Fig. l.—Photichthys argenteus ("Challenger Reports," vol. xxvii).
in. the black depths of the ocean, suddenly flashing out light from its luminous organs, and thus bringing into view any prey which may be near; while, if danger is disclosed, the light is again at once extinguished. It may be observed that the largest of these organs is situated just under the eye, so that the fish is actually provided with a bull's-eye lantern. In other cases the light may rather serve as a defense, some having—as, for instance, in the genus Scopelus—a pair of large ones in the tail, so that "a strong ray of light shot forth from the stern-chaser may dazzle and frighten an enemy." In other cases they probably serve as lures. The "sea-devil," or "angler" of our coasts, has on its head three long, very flexible, reddish filaments, while all round its head are fringed appendages, closely resembling fronds of sea-weed. The fish conceals itself at the bottom, in the sand or among sea-weed, and dangles the long filaments in front of its mouth. Other little fishes, taking them for worms, unsuspectingly approach, and themselves fall victims.
Several species of the same family live at great depths, and have very similar habits. A mere red filament would, however, be invisible in the dark, and therefore useless. They have, however, developed (Fig. 2) a luminous organ, a living "glow-lamp," at the end of the filament, which doubtless proves a very effective lure.
These cases, however, though very interesting, throw little light on the use of the muciferous system in ordinary fish, which, I think, still remains an enigma.
In some of the lower animals the nerves terminate on reaching the skin at the base of rod-like structures similar, in many respects, to the rods of the retina, or the auditory rods of the ear, and of which it is very difficult to say whether they are organs of touch or of some higher sense.
Round the margin of the common sea-anemone is a circle of bright blue spots, or small bladders. If a section be made, there
Fig. 2.—Ceratius bispinosus ("Challenger Reports," vol. xxvii).
will be found a number of cylindrical organs, each containing a fine thread, and terminating in a "cnidocil"; and, secondly, fibers very like nerve-threads, swelling from time to time with ganglionic expansions, and also terminating in a cnidocil. These structures, in all probability, serve as an organ of sense, but what impressions they convey it is impossible to say.
Some jelly-fishes (Trachynemadæ) have groups of long hairs arranged in pairs at the base of the tentacles (Fig. 3), which have been regarded as organs of touch, and it is certainly difficult to suggest any other function for them. They are obviously sense-hairs, but I see no reason for attributing them to the sense of touch.
The so-called eyes of the leech, in Leydig's opinion, which is confirmed by Ranke, are also developed from the supposed special organs of touch. The latter are much more numerous, as many as sixty being developed on the head alone. They are cylindrical organs, lined with large nucleated refractive cells, which occupy nearly all the interior. A special nerve penetrates each, and, after passing some way up, appears to terminate in a free end.
I may also allude to the very varied bristles and cirrhi of worms, with their great diversity of forms.
Among insects and Crustacea there are a great number of peculiarly formed skin appendages, for which it is very difficult to suggest any probable function.
The lower antennæ of the male in Gammarus, for instance, bear a very peculiar slipper-shaped organ, situated on a short
stalk: this was first mentioned by Milne-Edwards, and subsequently by other authors, especially by Leydig. The short stalk contains a canal, which appears to divide into radiating branches on reaching the "slipper," which itself is marked by a series of rings.
Among other problematical organs, I might refer to the remarkable pyriform sensory organs on the antennæ of Pleuromma, the appendages on the second thoracic leg of Serolis, those on the maxillipeds of Eurycopa, on the metatarsus of spiders, the finger-shaped organ on the antennæ of Polydesmus, the singular pleural eye (?) of Pleuromma, and many others.
There is every reason to hope that future studies will throw much light on these interesting structures. We may, no doubt, expect much from the improvement in our microscopes, the use of new reagents, and of mechanical appliances, such as the micro-tome; but the ultimate atoms of which matter is composed are so infinitesimally minute, that it is difficult to foresee any manner in which we may hope for a final solution of these problems.
Loschmidt, who has since been confirmed by Stoney and Sir W. Thomson, calculates that each of the ultimate atoms of matter is at most 50000000 of an inch, in diameter. Under these circumstances we can not, it would seem, hope at present for any great increase of our knowledge of atoms by improvements in the microscope. With our present instruments we can perceive lines ruled on glass which are 90000 of an inch apart. But, owing to the properties of light itself, the fringes due to interference begin to produce confusion at distances of 74000, and in the brightest part of the spectrum, at little more than 90000, they would make the obscurity more or less complete. If, indeed, we could use the blue rays by themselves, their waves being much shorter, the limit of possible visibility might be extended to 120000; and, as Helmholtz has suggested, this perhaps accounts for Stinde having actually been able to obtain a photographic image of lines only 100000 of an inch apart. This, however, would appear to be the limit, and it would seem, then, that, owing to the physical characters of light, we can scarcely hope for any great improvement so far as the mere visibility of structure is concerned, though in other respects, no doubt, much may be hoped for. At the same time Dallinger and Royston Pigott have shown that, as far as the mere presence of simple objects is concerned, bodies of even smaller dimensions can be perceived. According to the views of Helmholtz, the smallest particle that could be distinctly defined, when associated with others, is about 80000 of an inch in diameter. Now, it has been estimated that a particle of albumen of this size contains 125,000,000 molecules. In the case of such a simple compound as water, the number would be no less than 8,000,000,000. Even then, if we could construct microscopes far more powerful than any we now possess, they could not enable us to obtain by direct vision any idea of the ultimate molecules of matter. The smallest sphere of organic matter which could be clearly defined with our most powerful microscopes may be, in reality, very complex; may be built up of many millions of molecules, and it follows that there may be an almost infinite number of structural characters in organic tissues which we can at present foresee no mode of examining.
Again, it has been shown that animals hear sounds which are beyond the range of our hearing, and that they can perceive the ultra-violet rays which, are invisible to our eyes.
Now, as every ray of homogeneous light which we can perceive at all appears to us as a distinct color, it becomes probable that these ultra-violet rays must make themselves apparent to the ants as a distinct and separate color (of which we can form no idea), but as different from the rest as red is from yellow, or green from violet. The question also arises whether white light to these insects would differ from our white light in containing this additional color. At any rate, as few of the colors in nature are pure, but almost all arise from the combination of rays of different wave-lengths, and as in such cases the visible resultant would be composed not only of the rays we see, but of these and the ultraviolet, it would appear that the colors of objects and the general aspect of nature must present to animals a very different appearance from what it does to us.
These considerations can not but raise the reflection how different the world may—I was going to say must—appear to other animals from what it does to us. Sound is the sensation produced on us when the vibrations of the air strike on the drum of our ear. When they are few, the sound is deep; as they increase in number, it becomes shriller and shriller; but when they reach forty thousand in a second they cease to be audible. Light is the effect produced on us when waves of light strike on the eye. When four hundred millions of millions of vibrations of ether strike the retina in a second, they produce red, and as the number increases the color passes into orange, then yellow, green, blue, and violet. But between forty thousand vibrations in a second and four hundred millions of millions we have no organ of sense capable of receiving the impression. Yet between these limits any number of sensations may exist. We have five senses, and sometimes fancy that no others are possible. But it is obvious that we can not measure the infinite by our own narrow limitations.
Moreover, looking at the question from the other side, we find in animals complex organs of sense, richly supplied with nerves, but the function of which we are as yet powerless to explain. There may be fifty other senses as different from ours as sound is from sight; and even within the boundaries of our own senses there may be endless sounds which we can not hear, and colors, as different as red from green, of which we have no conception. These and a thousand other questions remain for solution. The familiar world which surrounds us may be a totally different place to other animals. To them it may be full of music which we can not hear, of color which we can not see, of sensations which we can not conceive. To place stuffed birds and beasts in glass cages, to arrange insects in cabinets, and dried plants in drawers, is merely the drudgery and preliminary of study; to watch their habits, to understand their relations to one another, to study their instincts and intelligence, to ascertain their adaptations and their relations to the forces of nature, to realize what the world appears to them—these constitute, as it seems to me at least, the true interest of natural history, and may even give us the clew to senses and perceptions of which at present we have no conception.