There are about 24,000 kinds of fish in the world. Two-thirds of people live in seawater, and the rest live in fresh water. There are 2,500 species in China, of which 100 is a kind of medicine. Common medicinal animals are hippocampus, syngnathus, Monopterus albus, carp, crucian carp, sturgeon (swim bladder glue), Pseudosciaena crocea (otolith is brain stone), shark and so on. In addition, it is often used as raw material for pharmaceutical industry. For example, the liver of cod, shark or ray is the main raw material for extracting cod liver oil (vitamin A and vitamin D). Hydrolyzed protein, cytochrome C, lecithin and cephalin can be extracted from various fishes. The liver and ovary of puffer fish contain a lot of tetrodotoxin, which can be extracted to treat diseases such as neuropathy, spasm and tumor. The bile of big fish can be extracted as "bile pigment calcium salt", which is the raw material for artificial production of bezoar.
Fish live in seawater or fresh water all their lives, and most of them have bodies and fins suitable for swimming. Breathe through the gills and prey on the upper and lower jaws. The beating heart is divided into atrium and ventricle. Blood circulation is single circulation. The appearance of spine and head makes fish develop into a vertebrate that can best adapt to life in water. This is because the depth of water is different, and the pressure is different everywhere. The sea level is 1 atmospheric pressure, while the deep sea area can reach 1000 atmospheric pressure. The salt content of fresh water and seawater ranges from 0. 00 1 ~ 7% from fresh water to salt water. In addition, with the different geographical environment, the temperature difference and oxygen content of water are also very different. Due to the diversity of these waters, water layers, water quality, biological factors and abiotic factors in water, the attitude structure of fish has undergone different changes to adapt to different external changes. Higher than rotifers.
Fish is the largest phylum of vertebrates. From the perspective of animal evolution, this class is the beginning of the jaw, so it is the most primitive and oldest class in the jaw. This is the largest taxon in the subphylum of vertebrates. As early as Devonian, many marginal branches have been derived and evolved into various complex fish. The existing fish are divided into cartilaginous fish and bony fish.
1. Chondroichthys. Compare OSTEICHTHYES
This strain is the lowest group of existing fish, with more than 200 species in the world and 140 species in China, most of which live in the ocean.
Its main features are:
① There is no hard bone for life, and the endoskeleton is composed of cartilage.
② Most of the body surface is covered with scales.
③ The branchial interval is well developed and there is no gill cover.
④ Oblique caudal fin. This line is divided into two subclasses, Claudiobranchia and Holocephala.
2. Skeletal fish (bony fish)
Bone fish is the largest fish group in the world, with more than 20,000 species, most of which live in seawater and some in fresh water.
Its main features are:
(1) The bones hardened into hard bones in different degrees.
② The body surface is covered with hard scales, round scales or comb scales, and a few species are degraded without scales. Mucous glands of the skin are well developed.
(3) The branchial septum is partially or completely degenerated, and the gills are not directly open to the outside world, but covered by the bony branchial lid. The water flowing out of branchial fissure is discharged through the rear edge of branchial lid, and there are many swim bladders.
(4) Fish tails are often straight, with original tails or crooked tails.
⑤ Most of them are fertilized in vitro, oviparous, and a few are abnormal.
The earliest fish was a round-billed jawless fish that appeared on the earth during the Cambrian period 450 million years ago. Fish are easy to distinguish from each other in appearance. They constitute the largest group of vertebrates: there are 22,000 species of fish among 50,000 species of vertebrates.
Not all animals that live in water are fish. For example, whales are mammals. However, all fish can adapt well to life in water. They move with their fins. Fish has two pairs of fins-pectoral fin and ventral fin, which are located in two parts of the body; There is also a tail fin, which grows at the tail; And depending on the species, there are one or two dorsal fins on the back and a gluteal fin on the buttocks. They have a bladder filled with gas, called swim bladder, which enables the fish to sink, float and keep their position in the water. Only rays and sharks do not have this organ. Fish also have gills for breathing, and most kinds of gills are covered by gill cover bones. Cheeks are located on both sides of the head and behind the mouth. They are used to filter the water swallowed from the mouth, get oxygen from the water, and then discharge the water from an opening called branchial fissure. The size of different kinds of fish varies greatly. Their bodies consist of three parts: head, trunk and tail. The skin is covered with scales of different sizes and numbers. There is an obvious line on each side called lateral line, which is a sensory organ and is used to determine the direction. The muscles of some teleost fish are separated by some tiny bones.
The first fish that appeared on the earth had a round jawless mouth, and now only 70 kinds of such fish exist, forming jawless fish. Among these fish, lamprey is the most famous. They have no scales, thin and round, much like eels. Lamprey attaches itself to other fish through its mouth with sucker, and feeds on the blood of its host. Other fish are divided into two categories: bony fish and cartilaginous fish.
A bony fish has a pair of bones. There are primitive bony fish in this fish, and only some bones are hard bones. For example, fish of the suborder Holoptera (including Echinodermata), lungfish and sturgeon (such as sturgeon) are different from the more evolved teleost in that the bones of teleost are completely composed of hard bones. As representatives of bony fishes, sea eel, sole and stickleback have different shapes, but they all have extremely symmetrical tail fins and are covered with tiny scales (with a few exceptions, including eels and some carp). Bony fish are divided into several types. Eels are some fish, and their larvae look very different from adults. Herring is a group of fish. Carp contains almost all freshwater fish. Perch and tuna are fish whose tail fins are supported by hard spokes. They are called "spiny fins" and constitute the largest bony fish.
The skeleton of cartilaginous fish is completely composed of cartilage and strengthened by calcium. This kind of fish is mainly sharks and rays.
Eel is a small fish with a flat body at birth and is called "Little Eel". When they are mature, they have long, smooth bodies, usually without scales, and continuous fins along their backs. Eels live in rivers and lakes in Europe and America. They migrate to the algae sea in the northeast of the North Atlantic West Indies, where they breed. They die after their cubs are born. On the way back across the Atlantic, the young eels showed their adult form, and on the way back, they also began to breed in the sargasso sea.
Herring lives in the North Sea, the English Channel and the Baltic Sea. Adult fish have a pale abdomen and a dark blue or nearly black back. Like sardines and herrings, they live in groups, sometimes thousands of fish live together. For every fish, this is an effective self-defense method, because for a predator, only one target can be selected among many fish. When attacked, the fish disappeared almost immediately.
There are thousands of kinds of carp, almost all over the world. These freshwater fish have large scales, and their teeth are not fixed on their chins, but on their throats. Their mouths can move forward to suck food. Many species of Cyprinidae mainly live in calm rivers, small ponds and lakes in Asia and Europe, and their shapes and colors vary greatly. Some species have only a few large scales (mirror carp) or almost no scales (grass carp). These fish breed easily, and breeders have created many varieties. Carp mainly feed on plants and invertebrates. The spawning season depends on the water temperature, not too cold (at least 20℃). Female fish lay hundreds of eggs, but most young fish become food for other fish and even adult carp at birth.
Spiny fins appeared about 60 million years ago. Perch is a typical representative of this kind of fish. Their fins are supported by hard and sharp spokes, and there are thorns on their huge tail fins. Perch lives in lakes and rivers in Europe and North America. They eat invertebrates and small fish, including their own young fish. Other species of these fish live in the ocean, such as tuna and swordfish. They are powerful swimmers and can swim 100 kilometers per hour. Tuna can weigh up to 500 kilograms and is a carnivore. In fish, they have a unique ability to keep their body temperature higher than the water temperature. Their species include Pacific albacore tuna and Mediterranean and Atlantic bluefin tuna.
Sharks and rays are the main representatives of modern cartilaginous fish, which may have appeared 465.438 billion years ago. As the name implies, their bones are made of cartilage. Cartilage is a flexible material, which will harden after calcium supplementation and be solid like bone. Chondroid fish thrive in temperate and tropical seas. They breathe through their gills in the water. Gills communicate directly with the outside world through several gill slits behind the head. There are about 550 species of cartilaginous fish, of which 370 species are sharks, and the others are basically composed of rays and rays.