The Organs of Sight— The eyes of fishes differ from those of the higher vertebrates mainly in the spherical form of the crystalline lens. This extreme convexity is necessary be cause the lens itself is not very much denser than the fluid in which the fishes live. The eyes vaiy much in size and somewhat in form and position. They are larger in fishes living at a moderate depth than in shore-fishes or river fishes. At great depths, as a mile or more, where all light is lost, they become aborted or rudimentary, and may be covered by the skin. Often species with very large eyes, making the most of a little light, or of light from their own luminous spots, will inhabit the same depths with fishes having very small eyes, or eyes useless for seeing, these being re tained as vestigial structures through heredity. Fishes which live in caves become also blind, the structures showing every possible phase of degradation. The details of this gradual loss of eyes, either through reversed selection or hypothetically through inheritance of atrophy produced by disuse, have been given in a num ber of memoirs on the blind fishes of the caves of Kentucky, Missouri and Cuba by Dr. Carl H. Eigenmann.
Many of the sharks possess a distinct nicti tating membrane or special eyelid, moved by a set of muscles. The iris in most fishes sur rounds a round pupil, without much power of contraction. It is frequently brightly colored, red, orange, black, blue or green.
In the lowest of the fish-like forms, the lancelet, the eye is simply a minute speck coated by black pigment, connected with the spinal cord by a short nerve. In the development of such a pigment-spot the vertebrate eye doubt less has its origin. In the hagfishes,. which stand next highest in the series, the eye, still incomplete, is very small and hidden by the skin and muscles. This condition is very dif ferent from that of the blind fishes of the higher groups in which the eye is lost through atrophy, because in life in caves, or under rocks, the organ is no longer necessary.
The Ear of the Fish.— The ear of the typical fish consists of the labyrinth only, in cluding the vestibule and three semi-circular canals, these dilating into one or more sacs which contain large, loose bones, the ear-stones or otoliths. There is no external ear, no tym panum and no Eustachian tube. The ear-sac on each side is lodged in the skull or at the base of the cranial cavity. It is commonly sur rounded by bone, but sometimes it lies near a fontanel or opening in the skull above.
The otoliths, two in each labyrinth, a-e usually large firm bone with enameled surface and peculiar grooves and markings. Each spe cies has its peculiar type, but they vary much in different groups of fishes. The sense of hear ing in fishes cannot be very acute and is prob ably confined chiefly to the perception of dis turbances in the water. Most movements of the
fish are governed by sight rather than by sound.
Voices of Fishes.— Some fishes make dis tinct noises, variously described as quivering, grunting, grating or Singing. The name grunt is applied to species of Hcensulon and related genera and fairly describes the sound these fishes make. The Spanish name •ronco or ron cador (grunter or snorer) is applied to several fishes, both scimnoid and ha-rnuloid. The noise made by these fishes may be produced by forc ing air from part to part of the complex air bladder, or it may be due to grating one on an other of the large pharyngeals. The grating sounds arise, no doubt, from the pharyngeals, while the quivering or singing sounds arise in the air-bladder. The midshipman, Porichthys notatus, is often called singing-fish, from a peculiar sound it emits. These sounds may pos sibly be useful to the species, but they are not well differentiated, nor have they been so in vestigated as to be well understood.
Sense of Taste.— It is probably certain that fishes possess a sense of taste, though it is little differentiated and is in some species located in the barbels about the mouth. The tongue is without delicate membranes or power of mo tion. In some fishes certain parts of the palate or pharyngeal region are well supplied with nerves, but no direct evidence exists that these have a function of discrimination among foods. Fishes swallow their food very rapidly, often whole; and mastication, when it takes place, is a crushing or cutting process, not one likely to be affected by the taste of the food.
Sense of Touch.— The sense of touch is better developed among fishes. Most of them flee from contact with actively moving objects. Many fishes use sensitive structures as a means of exploring the bottom, or of feeling their way to their food. The barbel or fleshy filament, wherever developed, is an organ of touch. In some fishes, as the moray, barbels are out growths from the nostrils. In the catfish the principal barbel grows from the rudimentary maxillary bone. In the horned dace and gud geon the little barbel is attached to the maxil lary. In other fishes barbels grow from the skin of the chin or snout. In the goat-fish and stumullet the two chin-barbels are highly specialized. In the cod-fish the single beard is little developed. In the gurnards and related forms, the lower rays of the pectoral are sepa rate and barbel-like. Detached rays of this sort are found in the thread-fins and in various other fishes. Barbels or fleshy flaps are often developed over the eyes and sometimes on the scales of the fins.