Definition of fish, what animal is a fish?

Fish has accompanied mankind for more than five thousand years and has formed an indissoluble bond with mankind. It has become an extremely important food and ornamental pet in human daily life. But what animal is a "fish"? However, few people know how to define fish. With the development of science, people's definition of fish has also undergone great changes.

Baiji Dolphin - Whale Nearly 500 million years ago, a major leap took place in the history of life on earth. The earliest fish-shaped animals appeared, which opened the prelude to the history of vertebrates, leading to the development of the animal kingdom and entering a new era. new historical stage. Real fish first appeared more than 300 million years ago. Throughout the long history, a large number of fish have survived, but have long since disappeared over time. The fish living on the earth today are just A very small number of species that appeared and evolved later.

Human beings have been able to identify species and give names to them a long time ago. The commonly referred to as "fish" includes all animals in the water, so many animals living in the water are given fish names, including whales, seals, giant salamanders (salamanders), etc. , cuttlefish, squid, octopus, starfish, seaweed, sponge, amphioxus, etc. are confused with fish. After all, those aquatic animals are the real "fish". The classification of "fish" has different definitions in different eras.

Alligator - Crocodile Two thousand years ago, the ancient Greek philosopher Plato (427-347 BC) defined fish as: "This type (fish) is created by completely unconscious things." . The Lord of Transfiguration thought that it was no longer worthwhile to give a breath of purity to this class, since they were the offspring of every sin and there was an unclean heart. The Lord of Transfiguration threw them into the water and caused them to pass through the deep of the sludge, to breathe the wonderful and pure air. This is how fish and oysters and all other aquatic animals are distantly separated as a punishment for the great sin of ignorance." Plato's views are steeped in creationism. Due to the development of modern science, this view has been completely rejected.

Giant Salamander - Salamander "Erya" in the early Han Dynasty of my country classifies animals into four categories: insects, fish, birds and beasts. Fish includes fish, amphibians, reptiles and other lower vertebrates, as well as whales, shrimps, crabs, Shellfish etc.

The 18th-century Swedish naturalist Linnaeus (1707-1778) founded modern taxonomy. In his book "Natural Systems", he divided the animal kingdom into mammals, birds, amphibians, fish, insects and worms Wait for 6 categories. In 1859, British biologist Darwin published the book "On the Origin of Species", giving birth to systematic taxonomy. Since then, the definition and scope of fish have been determined.

Which animals are "fish"? Modern taxonomists define "fish" as: vertebrates that live in water throughout their lives, breathe with gills, and swim with fins. Fishes include three major groups: Cystostomes, Chondrichthyes and Teleosts. There are about 26,000 known species of fish in the world. They are the most diverse category of vertebrates, accounting for about 48.1% of the total number of vertebrates. .Most of them live in the ocean, and there are about 8,600 species of freshwater fish. There are nearly 3,000 species of fish in my country, including about 1,000 species of freshwater fish.

Living in the coarse sand in the shallow waters along the coast of my country, there is a small, slender, translucent "fish" that is no more than 50 mm long, with pointed ends. Its scientific name is "Amphioxus". Although its appearance is It is a fish, but it does not belong to the fish category. Although the dorsal side of the amphioxus has a notochord, it has no spine structure, no bones in the body, no obvious head, and no heart. The characteristics of amphioxus that resemble fish are mainly reflected in the presence of notochord, neural tube and gill slits. Taxonomically, it is in the intermediate transitional group between invertebrates and vertebrates - holochordates of protochordates, also known as headless species.

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