Fishes
The term fish is applied to a variety of cold-blooded aquatic vertebrates of several evolutionary lines．Fish share certain features with other vertebrates．These features are gill slits (鳃裂) at some point in the life cycle, a notochord (脊索), or skeletal supporting rod, a dorsal hollow nerve cord, and a tail.
The study of fishes, the science of ichthyology (鱼类学), is of broad importance．There are many reasons why fishes are of interest to humans; the most important is their relationship with and dependence on the environment．A more obvious reason for interest in fishes is their role as a moderate but important part of the world's food supply．This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment．Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean.
Another practical reason for studying fishes is their use in disease control．As predators on mosquito larvae, they help curb malaria and other mosquito borne diseases．Fishes are valuable laboratory animals in many aspects of medical and biological research．For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions．Fishes have been especially important in the study of animal behaviour where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates.
There are aesthetic and recreational reasons for an interest in fishes．Millions of people keep live fishes in home aquariums for the pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them．To many, aquarium fishes provide a personal challenge, allowing them to test their ability to keep a small section of the natural environment in their homes．Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year．Interest in aquarium fishes and sportfishing support multimillion dollar industries throughout the world.
Fishes have been in existence for more than 450,000,000 years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat．In a sense, land vertebrates are simply highly modified fishes, for when fishes colonized the land habitat they became tetrapod (四足的) land vertebrates．The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform．to it．For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely．Air breathers have appeared in several evolutionary lines.
Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual．The brilliance of pigments may be enhanced by the suce structure of the fish, so that it almost seems to glow．A number of unrelated fishes have actual light-producing organs．Many fishes are able to alter their coloration, some for the purpose of camouflage, others for the enhancement of behavioral signals.
Fishes range in length from less than 10 millimetres to more than 20 metres and in weight from about 1.5 grams to many thousands of kilogr