The Encyclopedia Americana (1920)/Eyesight in the Lower Animals

EYESIGHT IN THE LOWER ANIMALS presents some curious differences from that in man and his nearer allies. The rudimentary eyes of the lower invertebrates can hardly be of more service than to convey an impression of the difference between day and night; many such come out of the ground, or rise to the surface of the sea, at night to sink again into darkness when the sun rises. In echinoderms, the starfishes for example, eyes are found that contain &ldquo;many clear oval bodies imbedded in pigment, which,&rdquo; Huxley says, &ldquo;appear to represent the crystalline cones of a compound eye.&rdquo; (See ). Among mollusks the organs of vision range from none at all in certain deep-sea species through all degrees of complexity to the highly developed eyes of the squid. Bivalves that creep about near shore, daily exposed between tides and subject to capture, have many eye-spots on the edge of the mantle, which are so sensitive that the shadow of a boat or of a man will cause instant closure of the shells. This is most noticeable in the scallops (Pecten), where a row of eyes glisten like jewels when the shell is open; and in the ark-shells (Arca), where a great number of eyelets are gathered in round groups forming compound eyes. In most univalve mollusks there are two eye's on the head at the base of the &ldquo;feelers&rdquo;; but in the land-snails the eyes are usually at their tips. Experiments show that snails are very short-sighted and see better in a subdued than in a bright light; but some seem to perceive subjects well several inches away. The eye of an octopus or a squid is very large and perfect, and as useful as that of a predatory fish which the squid resembles in habits. Among the trilobites and crustacea, also, the agile, prey-hunting species are provided with efficient compound eyes, usually set at the end of stalks that may be turned in any direction. These eyes are always compound, like those of insects &mdash; that is, consist of a honey-comb-like aggregation of somewhat modified simple eyes (facets) backed by one retina. Through each facet, it is believed, falls an exceedingly fine pencil of light, revealing a very small part of the field of vision. These combined fragments are supposed to form a sort of mosaic picture on the retina and in the insect's brain; and its field of view must depend on the number of facets and the approximation to the globular form of the whole external eye. The nature and value of insect-vision has been much discussed. Carpenter (&lsquo;Insects and their Structure and Life&rsquo; 1899), reviewing the controversy, concludes that the compound eye is especially adapted for perceiving sensations of light and motion rather than of form. There is reason to believe that such eyes do not perceive objects at a greater distance than six feet, while there is no doubt that they are able to appreciate color-sensations; in fact, the theory of the cross-fertilization of flowers by insects largely depends on this assumed ability. Dragon-flies, which have very large protruding eyes, of many

thousand facets, often approach close to person and hover there, evidently studying the details of the moving object (one's self) that has attracted their attention.

Vision among fishes is adapted to the medium in which they live, and varies from total atrophy of the eyes in subterranean waters and sea-abysses to eyes so large as to equal half the head. The relative size and position of the eyes in the head varies with habits in a bewildering degree, as fishes depend largely on eyesight for finding food and escaping enemies. Those that creep along the bottom, or dwell at intermediate depths, for instance, have the eyes on top of the head, looking upward; but to catalogue these adaptations would require a long essay. In most of the families the eyes are so situated in the side of the head that the vision is monocular &mdash; that is, only one eye can be used at a time for viewing an object. The species that chase fleeing prey, however, can look forward with both eyes. Such eyes need to change the focus rapidly, and this quick accommodation to distance is not effected by an alteration of the convexity of the lens, as in birds and mammals, but by a muscular change in its position with regard to the retina. The structure of the fish-eye is substantially the same as that of the human eye, with the important exception that in all deep-sea fishes only the rods (see ) exist in the retina, the cones being absent; the conclusion is that these fishes do not perceive color, which seems to be the special function of the cones. Furthermore, a difference in the retina of the deep-sea fishes and other creatures indicates that these animals are &ldquo;day blind,&rdquo; that is, have eyes adapted to the gloom in which they constantly live and are blinded by the glare of a strong light. It appears, however, that there is no regular decrease in size of the eye, from a small or normal form at the surface to the immense and well-developed eye that characterizes many of the fishes dwelling in the oceanic abysses. The pelagic fishes with largest, most efficient eyes, are those living at a depth of from one to 300 fathoms and possessing light-organs, while below that stratum both light-organs and eyes decrease in size until the deep floor of mid-ocean is reached, when again big-eyed species occur. The only present explanation of the latter fact is that those abysses are lighted by the glow of innumerable phosphorescent invertebrates, by whose light the fish seek their food.

Little need be said of the vision of amphibians and reptiles, which is probably rather dull and restricted. These animals, like many fishes, depend more on the sense of smell than on sight to guide their actions, and have, in addition, good hearing

Vision of Birds.&mdash; The power of vision attains its highest development among birds; and nowhere but in human faces is the eye so expressively beautiful or so exquisitely adapted to its service as an optical instrument. Birds possess the keenest and most farsighted vision, and also extraordinary power of swiftly altering the focus of the eye to changing distances, accompanied by astounding acuteness in mental calculation. A swallow or nighthawk sweeping and dodging in the air is catching insects almost invisible to our eyes and excessively agile; a

hawk chasing a swift-winged and quick-turning sparrow or a leaping grasshopper must have eyes that can follow exactly every movement. More wonderful is the work of an eagle or vulture, which while soaring so high in the sky that it appears to us a mere speck sees and defines an object on the ground that we could hardly notice or recognize at a hundred yards, and darts down upon it with the speed of a bullet. Imagine the farsighted keenness of this act and then the perfection of the apparatus by which the focus of the eye is changed in accord with the speed of the bird's descent, keeping the object always in clear view and stopping at precisely the right instant to escape collision. A hummingbird will dash past one's eyes like a gleam of light and halt at rest on a twig without fail. A woodcock or partridge will rush at high speed through a tangled wood and never touch a twig. These feats are evidence of the perfection of birds' eyes, especially in the faculty of accommodation to quickly varying distances.

Eyesight among mammals is relatively less important than among birds, or even to mankind, for their alert hearing and keen sense of smell give them much information. Vision among them varies with their manner of life, and is restricted to their needs in each case. The most farsighted and useful vision, probably, is that of the large grazers on the plains &mdash; deer, antelopes, giraffes, horses, etc., although they use but one eye at a time. The beasts of the chase, like owls among birds, have eyes in front, so that they observe with both at once, and of these perhaps the wolves, foxes and wild hunting dogs are best endowed. Here as elsewhere structure of the eye and quality of vision are in adaptation to the habitual needs of each kind of animal, and are always correlated with the power of locomotion.