In the sea are multitudes of fish forms arranged in many families. The myriad species of eels agree in having no ventral fins and in having the long flexible body of the snake. Most of them live in the sea, but the single genus (_Anguilla_) or true eel which ascends the rivers is exceedingly abundant and widely distributed. Most eels are extremely voracious, but some of them have mouths that would barely admit a pin-head. The codfish (_Gadus callarias_) is a creature of little beauty but of great usefulness, swarming in all arctic and subarctic seas. The herring (_Clupea harengus_), soft and weak in body, are more numerous in individuals than any other fishes. The flounders (fig. 116) of many kinds lie flat on the sea-bottom. They have the head so twisted that the two eyes occur both together on the uppermost side. The members of the great mackerel tribe swim in the open sea, often in great schools. Largest and swiftest of these is the sword-fish (_Xiphias gladius_), in which the whole upper jaw is grown together to form a long bony sword, a weapon of offence that can pierce the wooden bottom of a boat. [Illustration: FIG. 116.--The winter flounder, _Pseudopleuronectes americanus_. (After Goode.)] Many of the ocean fishes are of strange form and appearance. The sea-horses (_Hippocampus_ sp.) (fig. 117) are odd fishes covered with a bony shell and with the head having the physiognomy of that of a horse. They are little fishes rarely a foot long, and cling by their curved tails to floating seaweed. The pipefish (_Syngnathus fuscum_) is a sea-horse straightened out. The porcupine-fishes and swellfishes (_Tetraodontidæ_) have the power of filling the stomach with air which they gulp from the surface. They then escape from their pursuers by floating as a round spiny ball on the surface. The flying-fishes (_Exocœtus_) leap out of the water and sail for long distances through the air, like grasshoppers. They cannot flap their long pectoral fins and do not truly fly; nevertheless they move swiftly through the air and thus escape their pursuers. In its structure a flying-fish differs little from a pike or other ordinary fish. [Illustration: FIG. 117.--A sea-horse, _Hippocampus heptagonus_. (After Goode.)] For an account of the fishes of North America see Jordan's "Manual of Vertebrates," eighth edition, pp. 5-173, and Jordan and Evermann's "Fishes of North and Middle America," where the 3,127 species known from our continent are described in detail with illustrative figures. =Habits and adaptations.=--The chief part of a fish's life is devoted to eating, and as most fishes feed on other fishes, all are equally considerably occupied in providing for their own escape.