Fish are the oldest vertebrates. They inhabit almost all aquatic environments on Earth - from freshwater lakes and rivers to saltwater seas and oceans. Fish are ectothermic vertebrates that live in water all year round, use gills to breathe, and use fins to assist body balance and movement. Except for those animals that are called fish but are not fish, such as octopus, squid, and whales, there are about 20,000 species that have been identified, which are the most primitive and lowest group of the vertebrate subphylum.

There are two categories of fish belonging to the subphylum Vertebrates: Chondrichthyes and Osteichthyes.

The Chondrichthyes include 13 orders, 49 families, 158 genera, and about 837 species; China has 13 orders, 40 families, 90 genera, and about 202 species.

For example, the order Elasmobranch, the order Slits (the ancestral sharks), the order Sarcos (early freshwater sharks), the order Carcharhinus (the later sharks), and the order Rays (the flat-bodied sharks) Sharks), holocephalosaurs, bradyodontids, and chimaeras.

The teleost class has the following:

Lemonoptera

Characteristics: There are internal nostrils in the mouth; there are protofin-type even fins, that is, even fins It has a well-developed fleshy base, the fins are supported by segmented basal fin bones, and are covered with scales on the outside, taking the shape of fleshy leaves or whips; there are spiral valves in the intestines.

(1) Osteosquamata

***4 orders and 9 families disappeared in the Permian (about 150 million years ago)

( 2) Coelacanths

The speartail fish appeared in 360 million years and disappeared in 250 million years. In 1938, one was caught in the southwest Indian Ocean near South Africa, with a blue body. It was 1.1 meters long and weighed 50 kilograms. Local fishermen sent it to the museum. Museum staff Latimer peeled off its skin and asked Smith to identify it. He found it to be a spear-tailed fish and a living fossil. In commemoration of Latimer, it was named Latimerin, which is a research system. Excellent material for evolution.

Actinopterygii

Actinopterygii (also known as Teleostomi) is a bony fish of 28 orders produced in China. Cartilaginous Squamata: Sturgeons (sturgeons); Osteichthyans (Holosteichthyes): gars, bowfins; Teleosteus: most living fish.

Characteristics: The even fins generally have no fleshy blades at the base (except for polyfin fish), there is only one row of branch fin bones, and the fin rays are arranged in a radial pattern. No internal nostrils. Most species have swim bladders. Degeneration of gill septa. The cloaca does not exist, and the anus and cloaca are generally not located near the base of the pelvic fin (except for Acipensariiformes), and the anus opens in front of the cloaca. The body is covered with hard scales, round scales or comb scales, or is exposed without scales. No internal nostrils. No flippers.

(1) Ganoidomorpha

Characteristics: pelvic fins in ventral position, pectoral fins in low position, caudal fin crooked or truncated; most have larynx in the throat plates; most of the scales are rhombus-shaped hard scales; most of the swim bladder has spiral valves; the heart artery conus has 3-8 rows of valves.

(2) Clupeomorpha

Characteristics: The pelvic fins are located ventrally, with generally no less than 6 rays; the base of the pectoral fins is low, close to the ventral edge; the fins have no spines. Round scales.

(3) Anguillomorpha

Characteristics: elongated body, more or less eel-shaped. The pelvic fins are ventral or absent, and the dorsal and anal fins are usually very long and connected to the caudal fin. The body of the larvae resembles a willow leaf and undergoes obvious metamorphosis during ontogeny. This general order is divided into 3 orders: Anguilliformes, Saccopharyngiformes and Notacanthiformes. Only Anguillariformes are known to occur in my country.

(4) Cyprinomorpha

Characteristics: pelvic fin in ventral position, 1 dorsal fin. Usually the fins have no spines, but sometimes the dorsal fin, anal fin and pectoral fin have 1-3 spines, which are segmented pseudospines and are formed by ossification of the fin rays. Some species have adipose fins. The swim bladder has a tube leading to the digestive canal.

It has Weber's organ, in which the tripod bones are connected with the swim bladder, so this type is called bony swim bladder. Widely distributed in all continents of the world, most of them live in freshwater waters, especially tropical freshwaters. There are many species, about 5,000 species, many of which have important economic value.

(5) Percomorpha

Characteristics: pelvic fin in pectoral or throat position; fins generally have spines, body usually covered with comb scales, rarely exposed or covered with small bone fragments or Bone plates; many types of head bones have spines; the upper edge of the oral cavity is composed only of the maxilla; the swim bladder has no tube or no swim bladder, and rarely has a swim bladder with a tube.

(6) Atherinomorpha

Characteristics: pelvic fin in ventral position, sub-pectoral position, with 5-9 rays; pectoral fin in high position, base oblique or vertical; dorsal fin 1 or 2. The swim bladder has no tube. The body is covered with round scales. This general order consists of 3 orders, namely Cyprinodontiformes, Atheriniformes, and Beloniformes.

(7) Parapercomorpha

Characteristics: pelvic fin in subthoracic, thoracic or throat position. The swim bladder has no tube. Round scales or comb scales. The odd fins may or may not have spines, and some species have adipose fins. The caudal fin skeleton has a mid-axis subcaudal bone fragment. The general order Salmon Perch is divided into 2 orders, namely Percopsiformes and Gadiformes. Only Codidae are produced in my country.

(8) Batrachoidomorpha

Characteristics: short and thick body, flat or flattened sides, exposed skin, small spines or small bony plates; small gill openings located on the pectoral fins The pelvic fins on the outer ventral surface are at the pectoral or throat position; they are all bottom-dwelling carnivorous fish. There are 4 orders in this general order, 3 of which are produced in China, namely, the order of the sea mothfish, the order of the throat plate fish and the order of the anglerfish. Throatfish is known to be native only to Taiwan.

Brachioptera

Polypterygoid fish: The heart has an arterial cone and there are spiral valves in the intestine. The body is covered with diamond-shaped hard scales; the dorsal fin has 5-18 free finlets. The base of the pectoral fin is a movable fleshy leaf. Representative fish: polyfin fish.

Lungfish

Bipfin fishes: all ancient fishes (from the Devonian period 410-355 million years ago to the Triassic period about 248 million years ago) to 208 million years ago, all were ancient fishes)

Ceratodonts: Lungfish, whose swim bladders function as lungs, appeared in the early Triassic and continue to this day.