FISHES 239 a few enjoy sexual congress, and are ovovivipa- rous and viviparous, but the young are almost always left to themselves as soon as born. It is owing to the simultaneous development of great numbers of eggs deposited in the same locality, and to the instinct possessed by some species to keep in company, that fish occur in what are called banks and schools; these schools, composed of individuals kept together only by similarity of food and habits, and in which each one looks out for himself without regard to the wants of the rest, make long mi- grations from the sea to the rivers and back again, and from one favorite locality to another. At the time of laying the eggs, the migrating species generally approach the shores, and as- cend rivers, often coming thousands of miles ; year after year, at the same season, the fish appear in immense numbers. The migrations of the herrings, salmon, shad, smelt, mackerel, &c., afford well known instances of these phe- nomena. All fishes are of distinct sex. The testes vary much in form in the osseous fishes, and are remarkable for their enormous devel- opment in the breeding season, when they are called milt or soft roe. The ovaries in most osseous fishes are two elongated sacs, closed anteriorly, and produced posteriorly into short, straight, and wide oviducts, which coalesce before reaching the cloaca ; the greatly devel- oped ova are called the roe. There are several interesting points in connection with the de- velopment of fishes which will be better intro- duced here than in special articles. In most fishes it has been already stated that the exclu- sion of the ova or roe precedes fecundation, and that in a few (the sharks and rays especially) the ova are fecundated before exclusion ; when the embryonic membranes contract no adhe- sion to the uterine walls, the fish is called ovo- viviparous, and in such the embryo escapes from the egg before it quits the parent, while in the ovipara the ovum is expelled while the embryo is contained in it; when adhesion takes place by vascular interlacements, the species is said to be viviparous ; the great dif- ference between viviparous fishes and mammals is, that in the former the rupture of the mem- branes takes place long before birth, while in the latter this occurs at the moment of exclu- sion. 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 rea- son for their free or floating ribs. At the ap- proach of the breeding season the colors be- 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 maturity, and the male follows close to diffuse ie 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 323 VOL. vii. 16 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 (latrachus) 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 incu- bation is found in the pipe fish (syngnathw), in which the ova are transferred from the fe- male 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 va- rious stages of development even to the fish recently hatched ; eggs of two distinct species have been found in the mouth of a single indi- vidual. In the aspredos, or trompettis, the eggs are attached by pedicles surmounted by cups to the under side of the abdomen as far for- ward 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 di- vided into two groups: the first includes those in which the gestation is almost wholly ova- rian, as in embiotoca, anableps, blennius, &c. ; 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 Nat- ural History," vols. v. and vi.) has described the development of anableps Gronomi (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 papilla of the yolk sac carries on the func- tions of respiration and nutrition. In the em- biotocoidce of California the mode of develop- ment is similar; in E. lineata Girard found young three inches long and one inch deep ; in another genus of the group (Jiolconotus) he de- tected as many as 16 young about an inch long, which had evidently recently escaped from the egg shell ; the ovarian gestation here is some- what different from that in anableps, as the young ova are seen between the dividing mem- branes of the cvrary 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 impregna- tion. Many species of gadidce, as the cod, had- dock, whiting, and American hake, have been found to have a viviparous reproduction, the embryos being developed within the ovary,
