Naturalist.

Alexandra Herald and Central Otago Gazette, Issue 440, 18 August 1904, Page 2

Naturalist.

DEEP SEA FISH. THE oyos of doop-sea fish aro vory vanod : some have neither eyes nor sight; others have greatly enlarged eyoballs, so as to catch the loast glimpse of light. Their eyes tend cither to disappear or to be unusally efficient; but since no trace of sunlight can ponotrate to any great depth, and it is probably quite dark beyond a dopth of some two hundred fathoms, of what use can eyos be? Fish have been captured at a depth of nearly throe thousand fathoms, whore there must bo not only absolute stillness, but also total darkness—except for the fact that some of these deep-sea creatures are phosphorescent, and therefore luminous. This fact was first ascertained in the ' Challenger ' expedition. Since then, Mr Alcock of the Indian Marine Survey has found that some deep-sea crustaceans have a similar power, one large prawn quite lighting up a bucketful of water in which it was placed. Pish with large eyes have therefore a better chance of finding food (and mates); but they cannot wholly depend upon sight, since some have quite abandoned all attempts to sec. Some, again, have luminous organs on their head or body or tail, which are under their control, so that they can actually throw light at pleasure on their prey or extinguish it in times of danger. Thus the Angler, amongst others, attracts its prey by means of these* coloured lures or phosphorescent lights. It has been well said that these ' vast profounds of the deep have become a sort of alms-house or asylum, whereunto antiquated forms have retired, and amid the changeless environment, have dwelt for ages unaltered.' As is well known, the eyes of flat-fish — plaice, soles, &c. —are both on one side of the head. This is not, however, the case when these fish are born ; originally, like the majority of us, they have an eye on each side of the head ; but when they give up swimming about, and lie in hiding on one side at the bottom of the sea, the eye on the under side would be useless; it therefore gradually travels round the head till it is near the other eye, on the coloured side, which is directed toward the light. On the whole, fish have very large eyes, but not very keen sight, although they can perceive their prey or danger ! one very oddlooking tropical fish, that walks on its front fins, is much above the average in the way of sight: th is is doubtless necessary for its existence, since insets form its prey. Many fish, however, have an accurate vision for objects near at hand, as may be seen in the way they discover shrimps or other food when almost buried ; but few appear able to see objects at a greater distance than four feet in the water, and aboxvt three feet upwards. A man standing ' fifteen feet away can be seen by some ; but, owing to the refractive power of water, he would doubtless be greatly magnified. The pupils of a fish's eye do not, as a rule, alter in size with the changes in intensity of the ■ light; and in many, a change in the size of the eyelids marks the changes of the seasons; in some, the eyelids become so fat in the spawning season as almost to hide the eye ! Some sharks and a few of the lower mammals have a third eyelid or nictitating membrane, principally of use to clean the eyeball. Birds have very acute vision ; perhaps the most acute of any creature, and the sense is also more widely diffused over the retina than is the case with man ; consequently, a bird can see sideways as well as objects in front of it. A bird sees—showing great uneasiness in consequence—a hawk long before it is visible to man ; so, too, fowls and pigeons find minute scraps of food, distinguishing them from what appear to us exactly similar pieces of earth or gravel. Young chickens are also able to find their own food—knowing its position and how distant it is—as. soon as they are hatched ; whereas a child only very gradu- \ ally learns either to see or to understand the distance of objects.. Several birds—apparently the young of all those that nest on the ground—can see quite well directly they come out of the shell; but the young of birds that nest in trees or on rocks are born blind, and have to be fed. Burrowing rodents, such as rats and squirrels, as we might expect, and also insectivores (moles, &c), have a very rudimentary organ of sight. The walrus has not good sight either; at all events, out of the water it seems unable to see a man even when he is just in front of it, though it has keen enough hearing, and could smell him one thousand feet away, if to windward. When startled, the walrus rotates its eyes without moving its head, which gives it a very odd expression. Monkeys, as we should expect, have sight more like our own, and readily distinguish colour. In one instance, sugar-candy of various colours were provided for them; they invariably chose green first—perhaps because it was more like their usual herbaceous food—and then white ; no other colours were touched till these were all eaten.

It is, however, hardly necessary to speak of the sense of sight in the higher animals, as it is so much like our own, except to notice that few of them depend so much upon this sense as we do. This is very observable with cats and dogs, who, though they have keen sight, yet rely far more upon their senses of hearing and of smell. In most mammals and the higher vertebrates, as with ourselves, the eye consists of parts admitting light and concentrating it on an expansion of the optic nerve which -lines the back of the eyeball; sometimes one layer of tissue is modified into a coloured and light-reflecting surface.