during their first period of development float at the surface and are carried about by the currents. In all such cases the loss both of eggs and young must be very great, and to meet this loss such species usually produce enormous numbers of eggs. Thus a single, large cod may produce in a single year 10,000,000 eggs. On the other hand, in some species there is considerable care bestowed upon the eggs and young by the parents—this duty usually falling to the male. The sticklebacks (q.v.) are well-known instances. The male builds a nest of sticks, grass, etc., cementing them together with a sticky excretion, and guards the nest and eggs during incubation. In some of the Siluridæ, after the young are ready to leave the nest, the male may be seen leading the brood about, guarding it until the individuals are better able to shift for themselves. This instinct is found in other families. The cave blindfish (Amblyopsis) retains the eggs during incubation in its capacious gill-cavity. The seahorse develops a brood-pouch along the ventral side of the body, in which the eggs and young are harbored. Many marine fishes, like the shad and the salmon, ascend the rivers each season to spawn. These migrations may be for great distances, and against the greatest difficulties, such as rapids and falls. Such migratory species are known as anadromous fishes. The reverse process takes place in the case of the eel, which goes to the ocean to spawn.
The spawning season of most species is during the spring months. In the tropics, where the rainy and dry seasons alternate, these are determining factors in the time of spawning of certain species. Many species spawn during the colder months, for example the Salmonidæ. Many fishes show on the approach of the breeding season a noticeable sexual difference, the male being marked by more brilliant colors, or by the temporary growth of tubercles on the head and other portions of the body. Many species, however, do not exhibit this sexual dimorphism.
The eggs of fishes vary greatly in size and shape. The typical fish-egg is globular, more or less transparent, having a tough protective membrane, within which the yolk-laden egg proper lies. The yolk is present in sufficient amount to maintain the embryo until it can swim about and feed for itself. The outer protective membrane is very commonly sticky, to enable it to cling to stones, weeds, etc. Sometimes eggs stick together in clusters. In other cases the outer coat has tufts of fine filaments with which the egg fastens itself to weeds. The shark's eggs are inclosed in large, horny, purse-shaped cases within which the embryo is developed. The period of incubation is very various. In certain pelagic eggs the embryo emerges from the eggs in 24 to 48 hours after deposit, while in other cases, as the trout, the period extends over three to six months. See Egg; Embryology.
Food. The food of fishes includes all sorts of vegetable and animal matter and forms. Some are omnivorous. Others are exceedingly choice about their food, living almost exclusively upon certain species of Crustacea, for instance. Some, like the carp, are vegetarians, and the smaller fishes are the principal food of the larger, predaceous forms, like the trout and bluefish. Many species subsist entirely upon the minute organisms they can strain out of the water.
Distribution in Space and Time. About 10,000 species of living fishes are known. In their distribution fishes are almost coextensive with water. The greatest variety is found in the tropics. Many families are exclusively marine, others as exclusively of the fresh water, while many have representatives in both, or spend a portion of their time in each. Certain groups, like the Cyprinodontinæ, are distributed only along the shallow shore waters; others, like the sharks and bluefish, are pelagic, living on the high seas, and such usually have a wide distribution. The ocean depths have their peculiar fish fauna—species modified for these peculiar conditions, and unable to subsist at the surface. These species, living in darkness, often have no eyes, and many are phosphorescent. The coldest latitudes harbor their fishes. Some families, like the cod, are prevailingly distributed in colder waters, and certain species have been taken in lakes above the line of perpetual snow. Temperature is one of the important factors in determining the distribution of fishes. Deep-sea forms, where the temperature is uniform, have a wide distribution. The geological history of any region, with the changes in the river systems, etc., it has brought about, is another important factor. See Deep-Sea Exploration; Distribution of Animals; Cave Animals; Evolution.
Ancestry. The medium in which fishes live and the hard and almost indestructible nature of some portions of their skeleton, as their teeth, spines, and scales, would lead us to anticipate their frequent occurrence in the sedimentary rocks; but inasmuch as the soft parts of the animal are liable to speedy decomposition, the remains of fish must often exist in a fragmentary and scattered condition. Thus the teeth in the shark, the spine defense in the stingray, and the scales in the bony pike would survive the total destruction of the cartilaginous skeleton as well as the soft portions. Many quite complete casts of skeletons, however, have been obtained, so that not a little is known of the past history of the group. The earliest fishes occurred in the Upper Silurian. Remains of all of the main groups, excepting the higher teleosts, have been found from this period. Among the elasmobranchs the earlier forms were quite distinct from any now living, with the possible exception of the Port Jackson shark (q.v.). These forms flourished to the Triassic period, and in the case of the cestracionts to the Eocene. The recent elasmobranchs appeared in the late Triassic or early Jurassic, and were more abundant in the past than at present. The Dipnoi flourished in the Triassic. The ganoids were a dominant group up to Miocene times, but at present exist in mere remnants. The dominant fishes of to-day, namely, the higher teleosts, first appeared in numbers during the Jurassic and Cretaceous periods. These at the present day exhibit the greatest diversity in type. In times past the other groups presented this great variety of form, and it is mainly those species that retained the more generalized characters that survived and are present with us now. See Extinction of Species.
Economic Value. By far the most important use of fishes to man is in supplying him with food, and in some regions they form the principal means of subsistence. Some fishes, nevertheless, are unpalatable, and even poisonous to a greater or smaller extent. The skin of some cartilaginous fishes yields shagreen, and the air-blad-
