What a delightful sensation must it be to all but escape the eternal downpull of gravity, to float and turn and rise and fall at will, and all by the least twitch of tail or limb,—for fish have limbs, four of them, as truly as has a dog or horse, only instead of fingers or toes there are many delicate rays extending through the fin. These four limb-fins are useful chiefly as balancers, while the tail-fin is what sends the fish darting through the water, or turns it to right or left, with incredible swiftness.
If we were able to examine some inhabitant of the planet Mars our first interest would be to know with what senses they were endowed, and these finny creatures living in their denser medium, which after a few seconds would mean death to us, excite the same interest. They see, of course, having eyes, but do they feel, hear, and smell!
Probably the sense of taste is least developed. When a trout leaps at and catches a fly he does not stop to taste, otherwise the pheasant feather concealing the cruel hook would be of little use. When an animal catches its food in the water and swallows it whole, taste plays but a small part. Thus the tongue of a pelican is a tiny flap all but lost to view in its great bill.
Water is an excellent medium for carrying minute particles of matter and so the sense of smell is well developed. A bit of meat dropped into the sea will draw the fish from far and wide, and a slice of liver will sometimes bring a score of sharks and throw them into the greatest excitement.
Fishes are probably very near-sighted, but that they can distinguish details is apparent in the choice which a trout exhibits in taking certain coloured artificial flies. We may suppose from what we know of physics that when we lean over and look down into a pool, the fishy eyes which peer up at us discern only a dark, irregular mass. I have seen a pickerel dodge as quickly at a sudden cloud-shadow as at the motion of a man wielding a fish pole.
We can be less certain about the hearing of fishes. They have, however, very respectable inner ears, built on much the same plan as in higher animals. Indeed many fish, such as the grunts, make various sounds which are plainly audible even to our ears high above the water, and we cannot suppose that this is a useless accomplishment. But the ears of fishes and the line of tiny tubes which extends along the side may be more effective in recording the tremors of the water transmitted by moving objects than actual sound.
Watch a lazy catfish winding its way along near the bottom, with its barbels extended, and you will at once realise that fishes can feel, this function being very useful to those kinds which search for their food in the mud at the bottom.
Not a breath of air stirs the surface of the woodland pond, and the trees about the margin are reflected unbroken in its surface. The lilies and their pads lie motionless, and in and out through the shadowy depths, around the long stems, float a school of half a dozen little sunfish. They move slowly, turning from side to side all at once as if impelled by one idea. Now and then one will dart aside and snap up a beetle or mosquito larva, then swing back to its place among its fellows. Their beautiful scales flash scarlet, blue, and gold, and their little hand-and-foot fins are ever trembling and waving. They drift upward nearer the surface, the wide round eyes turning and twisting in their sockets, ever watchful for food and danger. Without warning a terrific splash scatters them, and when the ripples and bubbles cease, five frightened sunfish cringe in terror among the water plants of the bottom mud. Off to her nest goes the kingfisher, bearing to her brood the struggling sixth.
Later in the day, when danger seemed far off, a double-pointed vise shot toward the little group of “pumpkin seeds” and a great blue heron swallowed one of their number. Another, venturing too far beyond the protection of the lily stems and grass tangle of the shallows, fell victim to a voracious pickerel. But the most terrible fate befell when one day a black sinuous body came swiftly through the water. The fish had never seen its like before and yet some instinct told them that here was death indeed and they fled as fast as their fins could send them. The young otter had marked the trio and after it he sped, turning, twisting, following every movement with never a stop for breath until he had caught his prey.