Fishes have a brain and a system of nerves and sense organs varying according to rank, and outlining the higher developments of the nervous system as found in mammals. Of the sense organs the most peculiar are the small sensitive bodies scattered in various parts of the skin, fins and mouth, called "end buds," each at the terminus of a nerve fibril. These buds seem to carry the sense of feeling, and are said to be represented in mammals by the taste buds in our tongues. They are aggregated in a narrow band along the side of the fish, and in a maze on the side of the head, called the "lateral line," the course of which is plainly visible on many fishes, as for example, on the sunfish of brooks and ponds. This lateral line consists of canals in the skin, opening to the surface by pores, and reached by branches of large nerves. The use of the lateral line to the fish is not well known, but it is believed that its cells are of service in balancing the body. As blind fishes are able to avoid obstacles with the greatest ease when swimming, it is possible, in the opinion of Dr. Bridge, that these organs enable their possessors to appreciate undulatory movements in the water in the shape of reflex waves from contiguous surfaces or objects.
One feature of the lateral line on the head are the "auditory organs," varying with the kinds of fish, which contain semicircular canals, with otoliths, in the more or less complete form of an internal ear. Each is reached by the auditory nerve from the brain and is also connected with the air bladder in many cases. Whether this is a true organ of hearing in the ordinary sense, or whether it serves some other purpose, as, for instance, the regulation of the distension of the air bladder, is not known. The old question of whether fishes hear sounds made above the water is not yet answered scientifically; but it is probable that they can feel the jar of sounds made in the water, which is equivalent to hearing, as far as it goes. Fishermen have a saying that if you swear you won't catch any fish—a good precept, anyhow; but more effective is the care anglers take not to step heavily, nor to make loud, jarring noises, near the bank of the stream in which they mean to cast their lines.
The great majority of fishes have good eyesight, and the eyes themselves are similar in structure to those of the higher land animals; but it seems probable that the range of vision is short. The eyeballs are usually large in proportion to the size of the head—sometimes strikingly so—and are movable; while the situation in the head is naturally such as to give the most advantageous vision according to the habit of life. Thus those of sharks, and other predatory sorts that live by the chase, are well forward; while those of bottom-feeders, and especially rays, flatfish, anglers and the like, are in the top of the head, looking upward. Nocturnal species have the largest eyes, but the unfortunate cave fishes, whose whole life is, and has been for unnumbered generations, passed in the total darkness of caverns and underground streams, have lost the use of their eyes altogether, and the organs themselves have disappeared by atrophy.
Blindness is found also in oceanic families that dwell far below the penetration of daylight; yet many fish of the Stygian depths, which, so far as we know, never leave that region of utter blackness, possess big and apparently efficient eyes. Most of the blind or nearly blind sea fishes thus far obtained have been in hauls from a depth of about 1,200 fathoms. It is believed that the ability to see in deep-sea fishes is connected with the light-giving (phosphorescent) organs possessed by many of them, and with the fact that animals of all sorts on the sea bottom in deep water are luminous, and so reveal themselves to the predatory creatures that feed on them, while the fishes' own "lanterns" enable them to chase moving prey, avoid enemies, and find mates.
Fishes have efficient olfactory organs situated near the snout, and in the higher families they are in pairs and become true, but internal, nostrils. The sense of smell is strong, and perhaps more useful on the whole than the sense of sight, especially among the carnivorous species. Sharks seem to follow their prey by scent like hounds.
All these senses serve instincts related to the necessities of the individual and the race in each kind of fish. This is sometimes manifested in what appears to us as cunning means of safety or of provision for young; but discriminative intelligence is small in fishes, which probably are able to learn little more than that at certain places and times food may be had, as is illustrated in cultivated fish ponds, where the captives from infancy onward are fed regularly. Anglers tell of old trout that refuse year after year to be beguiled by their experiments in flies; but it is doubtful whether this is anything more than an increased wariness due to frequent disturbance. The remora is, or has been, used by the Caribs of the West Indies and the negroes of Zanzibar for catching sea turtles, a line being fastened to a captive and comparatively tame remora carried in the boat, and the fish turned loose as soon as a turtle is seen at the surface. The remora will make a bee line for the turtle and attach itself firmly to the shell so tenaciously that both animals may be dragged to the boat. It is to be noted that the fishermen see the turtle near by before they dispatch their living grapple, and it is doubtful whether the remora has any notion of what it is doing. It simply obeys repeatedly an instinct. This very low degree of intelligence is doubtless owing to the almost invariable environment of piscine lives, in which virtually nothing occurs to suggest any change in traditional habits or arouse into activity any rudiments of mind a fish may possess. Mental inertness is characteristic of aquatic animals of all kinds, as contrasted with the correlated activity of body and mind of land animals stimulated by varied and changeable surroundings.
The breeding habits of fishes furnish one of the most interesting chapters in their natural history, and many surprising facts have been learned within a few years in regard to the reproduction of marine species, of great value to the sea fisheries.
In all fishes the sexes are separate. As a rule females are larger than males, and more numerous. The size of the egg in any group depends on the amount of food yolk stored for the sustenance of the young, which must thrive by its absorption until it is able to eat by its mouth. The largest are the eggs of sharks, etc. (Elasmobranchii), which resemble fowls' eggs. The European dogfish, perhaps two feet long, has eggs an inch in length, each in a flattened leathery "purse" having tendrils at the ends that twine about weeds and anchor it like a rocking cradle. The similar egg capsules of skates, dropped on the sand, are common objects on all beaches. Elasmobranch eggs are deposited at intervals throughout the year and, as they are exposed to comparatively little danger, are few in number. In most other orders spawning, as the egg laying of fishes (and aquatic amphibians) is termed, is limited to a short period, the eggs are small, and the number of eggs produced is often enormous—five or six millions in a large cod, for example.
In the majority of Teleostomi—a group name embracing all the modern bony fishes—the eggs are voided broadcast into the water, the males at the same time emitting clouds of milt. These eggs are of two kinds, one that sinks and, often being glutinous, sticks to some object on or near the bottom, and is called "demersal"; and another that contains an oil bubble, making it so buoyant that it floats, and these latter are called "pelagic." The fertilization of such spawn must be accidental, but as the milt and the eggs sink or drift together the number that come into fertilizing contact is no doubt considerable. Nevertheless, an extremely small percentage ever reach the point of hatching, and still fewer survive to become mature, for in addition to unfavorable circumstances of water and temperature, every living thing, almost, in the ocean, including the parent fish themselves, is a devourer of the eggs and young of fish; and it has been said that the vast number of eggs dispensed by certain species, only a single pair of which on the average survives to maturity, is one of nature's methods of providing food for the inferior forms of marine life.