The bones of fish are composed of cartilage (cartilaginous fish) or bony fish (bony fish). There are four to seven gills on both sides of the skull, and the former has evolved into lower disintegrated bone. The fish's spine is connected to the skull, with ribs in the chest and long spines protruding from the spine in the back, tail and abdomen.

Skeletal fish usually have fishbones made of hardened tibia in their muscles. Fish use fins to control its direction and movement. Most fish have bony spines on their fins to strengthen them. The pectoral fin and the ventral fin are paired and connected by the muscles of the shoulders and buttocks. The dorsal fin, caudal fin and anal fin are connected to the spine. Some fish don't have spines on the fins between their backs and tails, but they can harden into horniness to strengthen their strength. Fish mainly rely on the swing of their bodies and tail fins when they move.

2. Skin

Fish has two layers of skin, the surface layer contains glands that secrete mucus, and the inner layer has many connective tissues, scales and pigment cells are also in this layer. The mucus in the outer layer helps fish to reduce resistance when swimming. Cartilaginous fish have no mucus, but tiny dentate processes on their skin have a similar effect.

3, blood circulation and breathing

The blood circulation of fish is closed, and its heart is relatively simple, which is located near the gills and consists of an atrium and a ventricle. Fish gill has many capillary lobules, through which oxygen dissolved in water is absorbed into the blood. The power of fish gills is very high (some fish can use oxygen dissolved in 70% water), which may indicate that the power of fish red blood cells is very high.

There is a horny membrane outside the gills of teleost fish. When a fish breathes, it opens its mouth and gill cover at the same time, so that water is sucked into its mouth. The membrane on the gill cover prevents water from flowing in this direction. When the mouth is closed, water can be squeezed out of the gills through the mechanism in the front of the mouth. Cartilaginous fish have no gill cover. They have to keep swimming with their mouths open and let the water flow through their gills.

Some bony fish (such as eels) have very small gills, and their gills can be kept moist on land for a period of time to breathe on land for a period of time. Other more or less amphibian fish have other respiratory organs: some fish can breathe oxygen in the air directly through their skin, such as mudskippers.

Some fish can suck air into their intestines, and their smooth intestinal walls can absorb oxygen from the air. Some fish have prominent organs that can be used as respiratory organs. The swim bladder of some fish is connected with their intestines, and their bubbles can also be used to help breathe oxygen in the air. The intestinal processes of lungfish have evolved into lungs.

4. swim bladder

The swim bladder is an expandable bladder organ in fish, and its main function is not breathing. Fish rely on the swim bladder to adjust their proportion. With the swim bladder, the fish can rise or fall slowly without exercise. Most teleost fish have swim bladder as an organ to regulate buoyancy. The swim bladder was originally an expansion of the intestine. Some fish, such as the swim bladder of carp, are still connected with its intestine, while others, such as the swim bladder of perch, are completely separated from its intestine.

If a fish wants to reduce its specific gravity, it will release dissolved gas in the blood into the swim bladder. Some fish use the oval shape in the swim bladder to inflate, while others use the intestine and the trachea connecting the intestine and the swim bladder.

Similarly, fish can re-dissolve gas into the blood to increase their specific gravity. The swim bladder can also be used as a vocal organ.

Cartilaginous fish, some underwater fish and bony fish that swim fast will sink to the bottom without swim bladder.

5. Nervous system and sensation

The nervous system of fish is relatively simple, the brain is relatively small, and there is no brain epithelium. Fish have a very good sense of smell. Their noses and mouths are not connected. Fish's ears are composed of closed vacuoles, and some fish's ears are connected to their swim bladder through movable bones. The lens in a fish's eye is not adjustable. They can only see things near.

They can feel ultraviolet rays. Fish living in the bottom of the water have a very good sense of touch, especially tactile cells on the epithelium of lips and tentacles. Fish has a special organ that can feel the current: lateral line. There is a line in the middle of their sides, which consists of a pit in the skin. In the pit, there are cells and hairs that can feel the change of water flow.

Fish may lack pain because they lack the necessary brain system and receptors.

6. Reproduction

The reproductive organs of fish are located above the intestine on one side of the body. Most fish mate in vitro, and both male and female fish excrete their germ cells into the water at the same time. The number of fish eggs can vary greatly. Sturgeon can give birth to millions at a time, while the number of sticklebacks that give birth to children does not exceed 100 at a time. In most cases, males will take care of their offspring.

Some fish have no fixed sex, and their sex changes with their partners, even many times in their lives. Some fish are fertilized in vivo, and most of these fish give birth to small fish directly without laying eggs.

Ecology and habitat

According to the habitat of fish, fish can be roughly divided into freshwater fish, marine fish and estuary fish in between. A few fish will migrate between fresh water and seawater at different stages of their life cycle. For example, salmon (Salmon) was born in fresh water, then moved to seawater to grow, and then returned to fresh water to breed.

Estuarine fish are euryhaline creatures, which can tolerate large salinity changes. Fish that can migrate between fresh water and seawater, such as salmon, are also halophytes.

There are fish in most large water systems in the world, but there are no fish in some lakes with too high salt content. In addition, some rivers and lakes are seriously polluted and there are no fish in them. Some fish are specially bred as ornamental fish.

Fish is an important food. The world catches 1 100 million tons of fish every year. Many fish are on the verge of extinction due to overfishing. A paper published in Nature from May 65438 to May 2003 reported that the total number of fish in the world's oceans today is less than 10% of that in 950. Sharks, cod and sardines are particularly threatened.