The sudden and great increase of the milt and roe is not compatible with a firm bony cavity such as would be formed by ribs and sternum; this explains the physiological reason for their free or floating ribs. At the approach of the breeding season the colors lie-come brilliant, as is familiarly seen in the bright red throat of the male stickleback; the female seeks to deposit her eggs in shoal water, where the heat and light of the sun may bring them to maturity, and the male follows close to diffuse the fecundating milt over them. It is well known that some fishes deposit their eggs in species of nests, as the stickleback, bream (po-motis), and lamprey; Aristotle mentions a fish of the Mediterranean, a species of gobius, as making a nest of seaweeds and depositing the spawn in it, the male keeping guard over the female and her young; the hassars, siluroid fishes of Demerara (callicthys), make nests of grass and leaves, and both sexes guard the eggs and young; the toad fish (batrachus) has been observed on the south shore of Long Island lying concealed in deep holes protecting its young, which attach themselves to stones by means of the yolk sac.

Another kind of incubation is found in the pipe fish (syngnathus), in which the ova are transferred from the female to a kind of marsupial pouch under the tail of the male, being fecundated during this process, and the cavity closing over them; when the young are hatched they follow the male, and return into the pouch at the approach of danger; the male hippocampus or sea horse has a similar subabdominal marsupial pouch. In some species of bagre, a siluroid fish from the rivers of Surinam, the females carry their eggs in the mouth, showing the young in various stages of development even to the fish recently hatched; eggs of two distinct species have been found in the mouth of a single individual. In the aspredos, or trompettis, the eggs are attached by pedicles surmounted by cups to the under side of the abdomen as far forward as the mouth, on the sides of the pectoral and ventral fins, and as far as the middle of the tail; after the eggs are hatched the pedicles are absorbed. Viviparous fishes may be divided into two groups: the first includes those in which the gestation is almost wholly ovarian, as in embiotoca, anableps, blennius, etc.; the second those in which the egg enters the oviduct before the development of the embryo begins, as in the plagiostomes.

Prof. J.Wyman ("Proceedings of the Boston Society of Natural History," vols. v. and vi.) has described the devolopment of anableps Gronovii (see Ana-bleps), in which he found the ovarian egg free in a distinct closed sac, as the mammalian ovum is in the Graafian vesicle; when the foetuses escape into the oviduct the gestation is carried on nearly to its completion in the ovisac, which becomes vascular, and by its apposition with the papillae of the yolk sac carries on the functions of respiration and nutrition. In the em-liotocoidoe of California the mode of development is similar; in E. lineata Girard found young three inches long and one inch deep; in another genus of the group {holconotus) he detected as many as 16 young about an inch long, which had evidently recently escaped from the egg shell; the ovarian gestation here is somewhat different from that in anableps, as the young ova are seen between the dividing membranes of the ovary while the foetuses are in course of development in the general cavity of the organ; it is not determined whether their ova leave the ovisac before or after impregnation.

Many species of gadida, as the cod, haddock, whiting, and American hake, have been found to have a viviparous reproduction, the embryos being developed within the ovary, thus confirming the supposition of many in- telligent fishermen. Internal impregnation is very general in the plagiostomes, and as this is more certain than the indiscriminate spawning of common fishes, the eggs are much fewer in number and of larger size, as in birds; the egg in its passage through the oviduct receives a dense corneous covering, so that the cases resemble oblong flattened pillows, often with long tendrils at the corners, in which the embryo is snugly coiled up; they become attached to objects floating near the surface, and are there developed by the influence of solar light and heat; from the researches of Prof. Wyman it appears that in the skates the eggs are fecundated in the ovary, and that the egg case is formed in advance to receive it as it descends. From these and other structural peculiarities Agassiz has separated the chimaerae, sharks, and rays from fishes proper, and elevated them into a class, the selachians. Many facts go to show that fishes undergo a kind of metamorphosis as well as insects.

August Muller has proved that the two genera hitherto considered characteristic of the cyclostome fishes are really different stages of the same animal; he has raised ammocetes from the egg of petromyzon, and watched the change of the former into the latter genus.-The usual mode of impregnation in osseous fishes, so analogous to the manner in which the fertilizing pollen is brought in contact with the stigmata of flowers, naturally suggested the idea of artificial impregnation; and this has been successfully practised both by naturalists for the study of embryology, and by fish breeders as a profitable branch of industry. (See Fish Culture.)-In most fishes the young when hatched are left to shift for themselves, and of course the greater number are devoured by larger fish, aquatic birds, and reptiles; many species devour each other; small mackerel are often found in the stomachs of larger individuals, when they are abundant; so that with all their fecundity the class of fishes does not multiply beyond the limits set by nature. Though fishes are cold-blooded, and the watery element is less affected by sudden changes of temperature than the air, there are external circumstances which limit their distribution both in depth and extent of surface.