The spinal cord extends from the brain to the tail, passing through the neural arches of the different vertebra when these are developed. In the higher fishes it is cylindrical and in elastic. In a few fishes (head-fish, trunk-fish) in which the posterior part of the body is shortened or degenerate, the spinal cord is much shortened, and replaced behind by a structure called cauda equina. In the head-fish it has shrunk into "a short and conical append age to the brain." In the cyclostomes and chimaeras the spinal cord is elastic, and more or less flattened or band-like, at least posteriorly.
The nerves of the fish correspond in place and function with those of the higher animals. They are, however, fewer in number, both large nerve-trunks and smaller nerves being less de veloped than in higher forms. The olfactory nerves and optic nerves may be regarded as outgrowths of the brain. The olfactory nerves, or first pair, extend through the ethmoid bone to the nasal cavity, which is typically a blind sac, with two roundish openings, but is subject to many variations. The optic nerves, or sec ond pair, extend from the eye to the base of the optic lobes. In cyclostomes these nerves run from each eye to the lobe of its own side. In the bony fishes or Teleostei each runs from the eye to the lobe of the opposite side. In the sharks, rays, chimaeras and ganoids the two optic nerves are joined in a chiasma as in the higher vertebrates. The other nerves arising in the brain need not be mentioned in detail.
A sympathetic system corresponding to that found in the higher vertebrates is found in all the Teleostei or bony fishes, and in the body of sharks and rays when it is not extended to the head.
The operations of the nervous system in fishes are essentially those of the higher verte brates, the instincts being relatively weak and the intellect or power of choice among com peting responses to external stimulus being ap parently wanting. All acts of the fish may be regarded as reflex, the results of external stim ulus or of stimulus arising within the body of the fish. These actions in each species tend to run in grooves or to be repeated in a special way for each species. These ways constitute the habits of the fish.
Organs of Reproduction.-- In most fishes the germ-cells are produced in large sacs (ovaries, testes) arranged symmetrically one on either side of the posterior part of the ab dominal cavity. The sexes are generally but
not always similar externally, and may be dis tinguished on dissection by the difference be tween the sperm-cells and the ova. The ovary with its eggs is more yellow in color and the contained cells appear granular. The testes are whitish or pinkish, their secretion milk-like and, to the naked eye, not granular.
In a very few cases both organs have been found in the same fish as in Serranus, the sea perch of Europe, but all fishes seem to be normally dicecious, the two sexes in different individuals. There are no external genital organs, but in some species a papilla or tube is developed at the end of the uro-genital sinus. This may exist in the breeding season only, as in fresh-water lampreys, or it may persist through life, as in some gobies.
The great majority of fishes are oviparous, the egg-cells being fertilized after deposition. The eggs are laid in gravel or sand or other places suitable for the species, and the milt or sperm-cells of the male is discharged over or among them in the water. A very small quan tity of the sperm-fluid may impregnate a large number of eggs. In a number of families the species are ovoviviparous, the eggs being hatched in the ovary or in a dilated part of the oviduct. In no case is a real uterus developed. In the case of viviparous fishes actual copula tion takes place, and there is usually a modifica tion of some organ to effect transfer of the sperm-cells. This may be the sword-shaped form of the anal fin in many top-minnows, the fin itself being placed in advance of its usual position. It may be an alteration of the struc ture of part of the anal fin as in the surf-fishes (Embiotocide); or, as in the Elasmobronchi, large bony organs (claspers) may be developed from the ventral fins.
In some viviparous fishes, as the rock-fishes and rose-fishes (Scorpcenide), the young fishes are very minute at birth. In others, as surf-fishes, they are relatively large and few in number. In the viviparous sharks, which con stitute the majority of the species, the young are large at birth and prepared to take care of themselves.