The characteristic of fish is that the color of the back is more protective in water than the color of the abdomen.

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.

The main characteristics of fish:

-Shape

-Exercise

-Skin and its derivatives

bone

digest

-Breathe

-Circulating fish blood.

-Excretion and osmotic adjustment

Reproductive system of breeding fish

-Nerves and senses

-Endocrine

1. shape

(1) spindle shape

Also known as the basic type, it is the body shape of ordinary fish, suitable for swimming in the water, and the whole body is spindle-shaped and slightly flat. Among the three body axes, the head and tail axis is the longest, the dorsal and abdominal axis is the second, and the left and right axis is the shortest, which makes the whole body streamlined or slightly flat to reduce the resistance when moving in the water, so this fish is good at swimming. Often inhabit the middle and upper water. It can be used for long-distance migration of carp, grass carp, shark and fish.

(2) Transverse flat type

Among the three body axes of this fish, the left axis and the right axis are the shortest, and the ratio difference between the head and tail axis and the back axis is not too much, forming a flat shape with left and right symmetry, making the whole body shape obvious and wide. Therefore, the swimming ability is worse than that of the spindle type, and it lives in the middle and lower layers of water. Long-distance migration is rare. Such as pomfret, butterfly fish, bream, mullet, swallow fish and so on.

(3) Among the three axes of flounder, the left and right axes are particularly long, and the back axis and abdominal axis are very short, which makes the body shape flat and slow, less flexible than the first two, and more benthic. Like ray, ray? ? ? ? And catfish.

(4) Rod type

Also called eel type. The tail axis of this fish head is particularly long, while the left and right axes are almost equal and short, making the whole body stick-shaped. Its swimming ability is better than that of lateral flat type and flat type. Suitable for digging holes in underwater soil and living in underwater sandstone. Such as eels, eels and many kinds of sea eels.

In addition, there are some fish that show special shapes because they adapt to the special living environment and lifestyle, such as seahorse, sea dragon, mola fish, river fish, flounder, box fish and so on. Fish of any size can be divided into three parts: head, trunk and tail. It is characterized by no neck, and the head and trunk are connected and fixed, which is one of the differences between fish and terrestrial vertebrates. The dividing line between the head and trunk is the rear edge of the gill cover (teleost fish) or the last pair of branchial cleft (cartilaginous fish). The trunk and tail are generally bounded by the posterior edge of anus or the starting point of gluteal fin, or precisely by the end of body cavity or the first caudal vertebral body.

take exercise

Fish's appendage is fin, which is an exercise organ for swimming and keeping balance. Fins are composed of branched fins and fins, which are divided into two types: one is horny fins, which are not segmented and branched, and are generated by epidermis and found in cartilaginous fish; The other is scale fin or bone fin, which comes from scales and is segmented, branched or unbranched, and is found in bony fishes. Fins are connected by thin fins. Bone fin can be divided into two types: fin thorn and soft thorn. Fin spines are formed by the deformation of a fin. They are hard spines that are neither branched nor segmented, and are only found in higher fishes. The soft strip is soft and jointed, and its distal branch (called branched fin) or unbranched fin (called unbranched fin) is formed by merging the left and right halves. Fins are divided into odd fins and lateral fins. Lateral fins are a pair of fins, including 1 pair of pectoral fins and ventral fins, which are equivalent to forelimbs and hind limbs of terrestrial vertebrates. Odd fins are unpaired fins, including dorsal fin, caudal fin and gluteal fin. The basic functions of dorsal fin and gluteal fin are to maintain body balance, prevent tilting and swaying, and help swimming. Like a rudder, the tail fin controls the direction and pushes the fish forward. Common fish have the above five kinds of fins, chest, abdomen, back, buttocks and tail. But there are a few exceptions, such as Monopterus albus without lateral fins, whose odd fins are also degraded; Eels have no ventral fins; Electric eels have no dorsal fins and so on.

(1) caudal fin

According to the relationship between the shape and position of the coccyx, the caudal fin can be divided into three types.

1) Circular caudal fin: The caudal fin has 1 leaf, and the caudal vertebra extends all the way to the posterior end of the caudal fin, which is divided into dorsal-ventral symmetrical caudal fin and pointed caudal fin, which is more common in the embryonic and larval stages of fish.

2) Oblique caudal fin: The caudal fin is divided into upper and lower lobes, and the end of the caudal vertebra is slightly bent upward and extends into the upper lobe of the caudal fin. The upper leaf is long, and the lower leaf is small and slightly prominent, forming a curved caudal fin with asymmetric inside and outside. Common in modern cartilaginous fish and some bony fish. Such as sharks, sturgeons and so on.

3) Orthomorphic caudal fin: divided into two leaves that are symmetrical up and down. The end of coccygeal vertebra only reaches the base of caudal fin, but slightly upturned, leaving traces of curved coccygeal vertebra. The caudal fin is completely symmetrical in shape, and the lower lobe is supported by the added lower coccyx piece. Orthomorphic caudal fin is one of the characteristics of higher fishes. According to the change of fin shape, it includes many kinds of fin shapes.

4) Prototype caudal fin: The end of caudal vertebra extends straight to the tail end, which is round, not as sharp as the round tail, and the upper and lower leaves of caudal fin are roughly equal. This is a primitive tail shape, which is found in Cyclocephala, and fish are only found in young fish.

(2) pectoral fin

It is equivalent to the forelimbs of terrestrial animals and attached to the chest at the rear edge of the gill cover. It has the functions of moving, balancing and mastering the direction of movement for fish. When the fish stops moving forward, the pectoral fin is used to control the balance of the fish; When swimming slowly, pectoral fins play the role of rowing; When driving at high speed, the pectoral fin is close to the fish, and when it is lifted, it can slow down and brake; When one side of the pectoral fin clings to the fish and the other side is lifted, the fish turns to the lifted side to help the caudal fin act as a rudder.

(3) ventral fin

It is equivalent to the hind limbs of terrestrial animals, and has the functions of assisting dorsal fin and gluteal fin to maintain the balance of fish and assisting fish to lift and turn. The position of ventral fins varies from fish to fish, and the ventral fins of cartilaginous fish are generally located on both sides of cloacal foramen. Similar in shape to pectoral fin, but slightly smaller. The ventral position of the pelvic fin of teleost is called the ventral position of the pelvic fin. This is a relatively primitive species, such as carp, salmon, catfish, herring and so on; The chest located in front of pectoral fin and behind gill cover is called ventral fin chest position, such as perch, yellow croaker and snapper; The throat between the cheeks is called ventral fin throat position, such as catfish and? Family fish. The chest position and throat position of ventral fin are advanced features of fish after evolution. These ventral fins with different positions are important symbols in the evolutionary history of fish and are of great significance in animal taxonomy.

(4) dorsal fin and gluteal fin

Mainly plays the role of balancing fish. But there are also some long fish, dorsal fins and anal fins that can help the body move and push the body forward quickly. For example, the dorsal fin of hairtail, the anal fin of electric eel, and the dorsal fin and anal fin of sea eel can all push the body forward. Another example is the special shape of the hippocampus, which also relies on the tiny dorsal fin movement to push the body forward. Fin type is a form of record, indicating the composition and number of fins. In Latin, the first letter of each fin represents the category name of the fin, such as dorsal fin "D", anal fin "A", ventral fin "V", pectoral fin "P" and caudal fin "C". Capitalized Roman numerals represent the number of spines. Arabic numerals indicate the number of soft strips, and the number range of spines or soft strips is indicated by "one". When spines are connected with soft strips, they are indicated by "one" and separated by ",". For example, fin carp: d .. ⅲ-ⅳ-17-22; Personal information15-16; vⅱ-8-9； A...ⅲ-5-6; C.20-22。

The above shows that carp has a dorsal fin, 3 ~ 4 hard spines and 17 ~ 22 soft strips; 1 hard spines and 15 to 16 soft strips in pectoral fin; There are 2 hard spines and 8 ~ 9 soft strips on the ventral fin; There are 3 hard spines and 5 ~ 6 soft strips on the gluteal fin; 20 to 22 soft tail fins. The fin type of perch is d-I-13; A..Ⅲ-7-8; p . 15- 18； Section five. The bass has two dorsal fins. The first dorsal fin consists of 12 hard spines without soft strips. The second dorsal fin includes 1 hard spines and 13 soft spines. There are 3 hard spines and 7 ~ 8 soft strips on the gluteal fin; 15 to 18 pectoral fin; Abdominal fin 1 hard thorn and 5 soft strips. The movement of fish is closely related to the changes of body shape and fins, and its swimming power mainly depends on the following three ways: ① using the muscles of trunk and tail to contract wave-like movement. (2) rely on the swing of the fin to paddle. (3) Fish can make use of the reaction force generated by spraying water backward from gill holes to move forward. Besides swimming, a few fish have a special form of exercise, that is, jumping or flying. For example, silver carp can jump out of the water obliquely and then fall vertically into the water. After the flying fish jumps out of the water, it can also open its wide pectoral fins, and it can fly up to about 300m in the air. Salmon can repeatedly jump over various obstacles in the river and migrate from the sea to the middle and upper reaches of the river to lay eggs. In addition, there are few fish that can crawl, such as? ? ? ? , bounce coating.

3. Skin and its derivatives

The skin of fish consists of epidermis and dermis. The epidermis is very thin and consists of several layers of epithelial cells and mucus layers. The epidermis is rich in unicellular mucus glands, which can continuously secrete viscous liquid and form a mucus layer on the body surface, which has lubricating and protective effects on fish, such as reducing skin friction resistance; Improve sports ability; Remove bacteria and dirt attached to fish. At the same time, it makes the body surface slippery and easy to avoid the enemy. Therefore, the epidermis is of great significance to the life and survival of fish. Below the epidermis is the dermis, which is rich in blood vessels, nerves, skin receptors and connective tissue, and there are pigment cells, luster cells and fat cells in the deep dermis and scales. There are three types of pigment cells: black, yellow and red. Melanocytes and yellow pigment cells exist in the skin of common fish. Red pigment cells mostly exist in the local skin of exotic tropical fish. Glorious cells contain guanosine crystals without pigment, which is highly reflective and makes fish appear silvery white. Some fish live in the depths of the ocean or in the dark water layer, and there is another skin derivative-luminescent gland cells, which can secrete phosphorus-rich substances and emit fluorescence after oxidation. ? ? ? Some species are.

There are many scales between epidermis and dermis, or in dermis. Fish scale is a unique skin derivative of fish, which is composed of calcium and covers the whole body or part (a certain part) of the fish surface. It can protect the fish from mechanical damage and external unfavorable factors, so it is called "exoskeleton". This is also one of the main characteristics of fish. The scales of existing fish can be divided into three types according to their appearance, structure and occurrence characteristics.

(1) The meibomian plate is composed of dermis and epidermis, including the basal plate of dermis evolution and the dentate part on the plate, that is, the round or rhombic basal plate of hard bone buried in dermis and the conical spine (tooth-shaped) protruding from epidermis. The surface of dentin is covered with enamel evolved from epidermis, and the center of dentin is pulp cavity. The whole medullary cavity is open at the bottom of the floor, and blood vessels and nerves penetrate into the cavity. Scales on the surface of sharks have the same occurrence and structure as teeth and should belong to homologous organs, so shark teeth are also called leathery teeth. The structure of fern scales is primitive and is found in cartilage scales.

(2) Hard scales are rhombic bone plate scales evolved from dermis, with a layer of calcified hard scales on the surface and special light, which is called scintigraphy. Hard scales are the most primitive scales in teleost fishes, such as eels and sturgeons.

(3) Bone scale is a round bone structure evolved from dermis, the front end of which is inserted into the scale capsule, and the back end of which is exposed to the skin in a free state, showing a compound tile arrangement. According to the shape of the free trailing edge, it can be divided into round scale and comb scale. The free trailing edge of round scales is smooth and blunt, which is common in lower teleost fishes, such as Cypriniformes and Silverfish. There are serrated protrusions on the trailing edge of comb scales, which are more common in high-grade fish such as Perciformes. Both round scales and comb scales have concentric rings on their surfaces, which are called annual rings. Like the annual rings of plant stems, the age, growth rate and breeding season of fish can be inferred from this.

On both sides of fish's body, there are one or several lines that evolve from individual nests into tubes, which are called lateral line scales. Each lateral line scale has a lateral line hole, which can feel the low-frequency vibration of water. The scales of teleost usually identify species according to their number, size and arrangement. The arrangement of scales is usually expressed by a fractional formula, which is called scale type: for example, the scale type of crucian carp is 28-30, which means that the lateral scales of crucian carp are 28-30, the scales on the lateral lines are 5-6, and the scales under the lateral lines are 5-7.

4. bones

Fishbone is divided into cartilage and hard bone according to its nature. Cartilaginous fish keep cartilage for life, because there is calcium precipitation in softness, it is also called calcified cartilage. The bones of teleost are mainly hard bones, which are divided into softened hard bones and periosteum according to different shapes: the hard bones formed by ossification on the cartilaginous primordia are softened hard bones, such as spine, ear bones and occipital bones; Hard bones formed by direct ossification of dermis and connective tissue are called membranous bones, such as frontal bone, parietal bone and branchial lid bone. Fish bones are divided into two parts according to different parts: axial bones and appendage bones.

(1) The axial skeleton is divided into skull and spine.

1) has the largest number of skulls: the skull of teleost fish is composed of about 130 bone fragments (referring to existing fish, the ancient primitive fish skull can be as many as 180), which is the largest number of brain bones in vertebrates. The skull of fish is divided into two parts: skull and pharynx.

① The skull of cartilaginous fish is a cartilaginous cavity to protect the brain, with simple structure, no boundary and suture, and only the back of the skull is covered by a membrane. This kind of skull is called soft skull. The soft skull of cartilaginous fish is ossified into several occipital bones, ear bones, sphenoid bones, ethmoid bones, and membranous skull parts such as nasal bones, frontal bones, parietal bones and vomeroid bones, so the structure is very complicated. The skull of teleost is composed of many bone fragments, which constitute the main part of the skull.

② In vertebrates, the pharyngeal arch differentiates into the upper jaw and the lower jaw, and the well forms the pharyngeal skull. Among fish, the pharyngeal cranium is the most developed, which is formed by seven pairs of ">"-shaped pharyngeal arches. The first pair expands into a jaw arch. The posterior segment of the jaw arch is called palatal cartilage, and the abdomen is called Michael cartilage. They form the upper and lower jaws of cartilaginous fish. The appearance of the upper and lower jaws is more advanced than that of cirrhosis, and they can take the initiative to ingest food. However, teleost evolved into hard maxilla and maxilla, replacing cartilaginous maxilla (palatal cartilage), and Maxwell cartilage evolved into articular bone, odontoid bone and horn bone of cartilaginous hard bone. The second pair of lingual arches consists of bilateral lingual cartilages, angular lingual cartilages and central and basal lingual cartilages, which mainly support the tongue and also help to support the upper and lower jaws. The third to seventh pairs are branchial arches, which support gills and branchial septa.

2) The spine replaces the notochord: The spine of fish is connected by many vertebrae into 1 columnar bone to replace part or all of the notochord, which has the functions of supporting the body and protecting the spinal cord and main blood vessels, and is more advanced than the round mouth. Fish's spine is characterized by inward depression on both sides, which is called biconcave vertebral body or biconcave vertebral body. It is unique to fish, and residual notochord can be seen in the gap between two adjacent vertebral bodies and in the tubule running through the vertebral bodies. Vertebrates started from fish, and the basic structure of the spine has been formed. The vertebrae of cartilaginous fish and teleost fish are divided into vertebral body, medullary arch, medullary spine, pulse arch and pulse spine. Among them, the vertebral body is the main part, and the ribs are connected with the transverse process of the spine, and the ribs of teleost fish are mostly developed.

(2) The appendage is fin bone.

Appendages are divided into odd fin bones and lateral fin bones. The dorsal fin, gluteal fin and caudal fin bones in odd-numbered fins are all supported by the fin supporting bones (radial fin bones) inserted into muscles, and the fin supporting bones of teleost fish are also called fin supporting bones. The skeleton of the lateral fin consists of two parts: the skeleton with shoulder strap and waist bone and the fin bone (fin bone and fin strip). In fish, all appendages are not directly connected with the spine except the shoulder strap of teleost fish, which is one of the characteristics of fish, which is determined by the swimming mode of fish.

digest

The digestive system of fish consists of digestive tract and digestive gland. The digestive tract has gastrointestinal differentiation, and the pancreas is obvious. Fish live in water all their lives, so their digestive organs and feeding habits are adapted to life in water. The oral cavity is located between the maxilla and mandible, and there are no salivary glands in the oral cavity. Fish has real teeth in the oropharyngeal cavity, which can take the initiative to eat and prey, and is more advanced than cirrhosis. The teeth on the jaw of Cladosporidae are transformed from peltate scales, and the teeth of teleost are divided into oral teeth and laryngeal teeth according to different parts. Fish that generally feed on plankton have thin and fluffy teeth arranged in a toothed belt; Carnivorous fish have large conical teeth, canine teeth, molars or incisors; The teeth of omnivorous fish are cut, ground, brushed or grooved. Fish's teeth have the function of cutting and crushing food. Gill harrow is attached to the inner edge of gill arch of most fish, which plays a role in protecting gill and pharynx of fish and filtering food. The shape, position and number of fish teeth and gill rakes are often used as one of the basis for fish classification.

breathe

Among the spinal propellers, only fishes and sclerodermata are aquatic animals that breathe with gills all their lives, but the gills of fishes are formed by ectoderm, and the gills of sclerodermata originate from endoderm. Fish generally have 5 pairs of branchial arches (6 ~ 7 pairs in a few fish) and 5 branchial fissures on both sides of pharynx. Branches are mainly composed of gill arch, gill septum and gill flap. The branchial arch plays a supporting role. A gill rake is attached to its inner edge, and blood vessels entering and leaving the gill arch pass through it. The outer edge of branchial arch is branchial septum, which protrudes forward and backward to form branchial meridian. Numerous gill meridians are closely arranged into comb-shaped gill flaps, and numerous small protrusions on gill filaments are called gill lobules, which are places for gas exchange. Branchlets are covered with capillaries, and blood eventually flows into sinus cavity. The wall of sinus cavity is composed of connective tissue, which plays a supporting role. The surface layer of branchlets is a monolayer of epithelial cells, so the fish gills are bright red. The gills of teleost are primitive, with branchial cleft openings in the body and well-developed branchial septa. There are 1 half branches before and after. These two hemibranchials are collectively called holobranchia, and the outer side is protected by gill cover. The inner side under the gill cover is the gill cavity or gill chamber, which opens backward and outward and has a total gill hole. A soft gill cover membrane extends from the rear edge of the gill cover, which can tightly seal the gill hole. Chondroid fish has 4 full gills, 1 half gills, * * * 9 half gills, and no gill cover.

Fish not only breathe through gills, but also have organs that assist breathing, such as loach, which breathe through intestines by swallowing gas. Bullet fish, catfish and so on. Can breathe through the skin; Monopterus albus can breathe through the mouth; Black fish, catfish, etc. Gillfold breathing can be performed; Lung fish, etc. Breathe with the swim bladder. Fish have two nostrils, but they can't pass through the mouth (except lungfish and total fin).

The swim bladder is a membranous bundle located between the digestive tract and the kidney on the back of the body cavity, which is different from the digestive tract during embryonic development. Its shape varies from fish to fish, and it has one, two or more chambers. The main function of swim bladder is to adjust the ups and downs of fish or stay in a certain water layer. When the volume of the swim bladder increases, the proportion of fish in the water decreases and the fish floats. When staying in a certain water layer, the swim bladder needs to release some gas. When the volume of swim bladder decreases, the proportion of fish in water increases and the fish sinks. When you need to stay in a certain water layer from shallow to deep, you need to inhale some gas. In a word, the increase or decrease of gill gas is related to the pressure in water. The change of swim bladder volume is a slow process, so swim bladder fish should only live in a relatively fixed water layer. Living in the deep sea, rapids or benthos, or fish that swim fast, the swim bladder has lost its function in their lives. For example, sharks, mackerel and tuna swim very fast, but there is no swim bladder. Therefore, they must always be in motion and must stop at the bottom of the water. Another kinetic energy of the swim bladder is gas exchange. Cartilaginous fish and some bony fish use swim bladders to help them breathe. For example, African multi-fin fish exchange gas with lung-like swim bladder with 1 in dry season. Lungfish, finches and eels can also breathe with the swim bladder.

7. Promote the blood circulation of fish

It is a single cycle, and the heart is mainly composed of a venous sinus, an atrium and a ventricle. The heart acts as a pump in blood circulation. Its contraction presses blood (hypoxic blood) into the abdominal aorta and sucks blood from the back of the venous sinus when it relaxes. The blood entering the abdominal aorta goes forward under the pharynx, branches to both sides to form an arterial arch, and extends backward along the branchial bundle. The blood vessels entering the branchial lid from the arterial arch branch are the input arteries, and the blood vessels leaving the branchial lid are the output arteries. The inlet artery and outlet artery are connected by branchial artery capillaries, where gas exchange takes place. Oxygen-containing fresh blood flows through the branchial artery, flows into the dorsal artery through the superior branchial artery on the back of the branchial bundle, and then distributes to various parts of the body and internal organs, including the head artery, abdominal artery, renal artery and tail artery. The capillary network in these parts inputs the cephalic venous blood into the anterior main vein, and the two main veins merge into a common main vein. The capillary network of another group of organs (digestive tube wall) inputs venous blood into hepatic portal vein. Blood in hepatic portal vein and blood in hepatic artery pass through hepatic capillaries and eventually flow into hepatic vein. Blood from hepatic vein and main vein enters venous sinus and finally flows back to the heart, thus completing blood circulation. Bone fish also have arterial balls and can't beat. Cartilaginous fish has an arterial cone, which can automatically contract rhythmically with the ventricle. The function of the arterial ball and the arterial cone is to make the blood flow into the abdominal aorta evenly, so as to reduce the pressure caused by the strong beating of the heart on the branchial blood vessels. The blood circulation of fish is non-mixed blood circulation, the blood oxygen content in pulse is high, and the circulation efficiency is higher than that of mixed blood circulation. However, the heart of fish is very small, accounting for only 0.2% of the body weight, while the heart of mammals accounts for 0.59% of the body weight, and the heart of mullet is bigger, accounting for 0.82% of the body weight. So fish have low blood pressure and slow blood flow. For example, the average blood pressure of shark abdominal artery is 28 mm Hg. In this way, the metabolism of fish in water is lower.

8. Excretion and osmotic adjustment

The excretion of metabolites in fish is completed by kidneys and gills. The urinary organ is the kidney, and the fish's kidney is 1 long purple-red strip, which is located at the back of abdominal cavity and belongs to the middle kidney. In excreting waste, the main function of mesonephros is to form urine. Metabolites, water and nutrients dissolved in the blood are filtered by the glomeruli in the kidney, most of which (such as glucose, amino acids and sodium, calcium, magnesium and chlorine plasma) return to the blood, and the remaining filtrate and excess harmful substances form urine, which is excreted through the ureter. Besides the kidneys, gills secrete nitrides and salts, such as ammonia and urea. Experiments show that the nitrogen-containing substances secreted by gills of carp and goldfish are 5 ~ 9 times as much as those secreted by kidneys. In addition to the urinary function, the kidney of fish can also regulate the infiltration of water and salt in the body, because fish live in fresh water and seawater, and the external environment is usually not isotonic with tissue fluid and blood in the body. The salt concentration in seawater is above 3%, and that in fresh water is below 0.3%. If fish live in such an environment, it may cause dehydration or water absorption. However, this is not the case. Fish can still live in such water for a lifetime, which is mainly compensated and regulated by the regulation of kidney waste and some special cells in gills. The kidney of freshwater fish consists of a large number of large renal corpuscles and glomeruli. When the concentration of their body fluids and blood is higher than the water environment, the kidneys can continuously excrete urine (excessive water in the body), and at the same time, the salt-absorbing cells in the gills supplement the salt in the blood to maintain the water-salt balance of freshwater fish. On the contrary, because the salt concentration in blood and body fluids is much lower than that in seawater, marine fish have a tendency to continuously permeate water from the body to the outside. In order to adapt to the environment, marine teleost will swallow a lot of seawater, and a large amount of salt contained in the swallowed seawater is excreted by some salt-secreting cells in the gills. At the same time, in order to prevent water loss in the body, the glomeruli of marine fish often degenerate or disappear completely. Reduce the amount of urination and make it isotonic with body fluids. Therefore, the balance of water and salt in the body can be adjusted and maintained in these ways.

Some fish can swim from the middle reaches of the sea to Hanoi or from the middle reaches of the river to the sea, and can quickly adapt to the water environment with different salt concentrations. For example, salmon swims from the sea to the Tamsui River to breed; Eels swim from fresh water to the ocean to breed, and so on. Why can these fish adapt to the water environment with different salt concentrations quickly? How to adjust the osmotic pressure in the body? This is a very complicated problem, which needs further study.

9. Reproductive system of breeding fish

It consists of gonads and reproductive ducts. The gonad includes testis and ovary, and the reproductive tract consists of vas deferens and fallopian tubes. The appearance of reproductive duct is a step further than cirrhosis. Most fish are hermaphroditic and lay eggs. Most of them are in vitro fertilization. The gonads of female fish are ovaries, which are usually flat and banded, blue-gray, yellow, pink and other colors, and can occupy most body cavities when they grow up in the breeding season. The gonads of male fish are generally white linear testicles, which will still increase during the breeding season, and are called fish white, which is the place where sperm are produced. The gonads of cartilaginous fish and lower bony fish are exposed. The gonads of higher teleost fish are closed and wrapped by fascicular membranes differentiated from peritoneum, forming cystic ovaries or cystic testes. In addition, a few fish are hermaphroditic, such as many species of Eriocheir sinensis, which can fertilize themselves. Monopterus albus can be reversed, that is, the gonads from embryo to adult are ovaries and can only produce eggs. After the adult lays eggs, the ovary gradually transforms into the testis, producing sperm, thus becoming a male.

There are four ways of fish fertilization and development: ① in vitro fertilization and in vitro development. ② In vitro fertilization and in vivo development. For example, the male of Tachysurusbarbus in the genus Silurus stops eating during reproduction and swallows fertilized eggs into his stomach to hatch. ③ In vivo fertilization and in vitro development. Before spawning, the male fish passes through a special junction organ. Such as fin feet, short pipes, etc. The sperm flows into the reproductive hole of female fish, and the eggs are excreted in vitro shortly after fertilization in vivo, such as tiger sharks in cartilaginous fish. (4) Fertilization and development in vivo. For example, bony fish such as cartilaginous fish and wicker fish of the family Lepidoptera begin to develop after fertilization. For example, carp fertilized eggs can hatch in a week at 20℃, which is called incubation period. The newly hatched young fish is about 65438±0.2mm long, with transparent body, pigment, unhardened bones, undeveloped fins and yolk sac in the abdomen, which is called the young fish stage. When the yolk sac contracts, the young fish begin to eat and grow into a fish shape after the growth period (the third stage). Carp takes about 2 ~ 3 years from juvenile to adult, and its life span can reach several decades.

10. Nerve and sensation

(1) The nervous system of fish is mainly divided into central nervous system and peripheral system including brain and spinal cord. Like other vertebrates, fish's brain is divided into five distinct parts, but overall it is small and primitive, because some teleost fish have no nerve cells on the back of their brains, only epithelial tissues. The spinal cord is cylindrical, milky white, and has obvious segments. Each segment sends out efferent and afferent nerves, which plays the role of conduction and communication with spinal nerve, sympathetic nervous system and brain. The peripheral nervous system includes cranial nerve and spinal nerve. Like amphibians, there are 10 pairs of cranial nerves, namely olfactory nerve, optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, auditory nerve, glossopharyngeal nerve and vagus nerve. Other vertebrates have 12 pairs of cranial nerves. Spinal nerves are nerves from both sides of the spinal cord and heal at the dorsal root and ventral root. The dorsal root contains sensory nerve fibers from sensory organs or dorsal ganglia, which enter the spinal cord, so it is also called sensory root. The anterior root contains motor nerve fibers originating from the spinal cord and leading to all parts of the body, also known as motor roots. Like other vertebrates, fish combine sensory roots and motor roots at the medullary arch to form mixed nerves, which is more advanced than most jawless animals that do not combine sensory roots and motor roots to form spinal nerves. The mixed nerve of fish is divided into three branches: the dorsal branch is sensory nerve, which is mainly distributed in the skin, and the muscle part is motor nerve; The abdominal branches are mainly motor nerves, which are distributed in muscles, and some of them are sensory nerves, which are distributed in the skin; Visceral branches reach the sympathetic ganglion and communicate with the sympathetic nervous system. Although fish have sympathetic nerves and parasympathetic nerves belonging to the autonomic nervous system, they are quite primitive, indicating that the status of fish in vertebrates is still very low.

(2) Sensory organs The sensory organs of fish include the sense of smell, sight, hearing, taste and the side organs peculiar to aquatic vertebrates. The difference between the sensory organs of fish and terrestrial vertebrates lies in:

1) fish's eyes have poor vision: they can't see far in the water, and their lenses are spherical and inelastic, and their corneas are flat. In addition, most fish have no eyelids and lacrimal glands, so fish's eyes are often open and can't be closed. There are only a few fish that can climb ashore from the water, and the surface is limited, such as mudskippers.

2) There are no ear marks on the fish body surface, only the inner ear: the inner ear has ear spots (feeling sound) and otoliths (adjusting balance). Bony fish usually have three otoliths, which grow with age. Therefore, this stone can be used to study the age and growth of fish.

3) Fish-specific lateral line: it is a longitudinal pipe extending in the trunk and tail, which forms a lateral line organ with the branches of the pipe and covers the head. This organ can detect low-frequency vibration, thus judging the direction and size of water waves, sensing the changes of water flow direction and pressure, and the activities of surrounding creatures. Whether there are obstacles in the water and so on. The lateral line is innervated by vagus nerve, and the lateral line of the branch of the head is innervated by nerve.

1) pituitary gland

Located on the ventral surface of diencephalon, it is connected with the bottom of the third ventricle (diencephalon) through the funnel handle. The pituitary gland of teleost consists of anterior lobe, mesenchymal lobe, transitional lobe and nerve part. The first three parts are called adenohypophysis or main lobe, and the nerve part is called neurohypophysis or posterior lobe. Behind the front leaf is a mesenchymal leaf, and behind the mesenchymal leaf is a transitional leaf. Pituitary gland is the most important endocrine gland with 1. Hormones secreted by pituitary gland act on various tissues of the body, and regulate other endocrine glands, such as gonadotropin, which can promote gonadal maturation and ovulation. In addition, hormones secreted by pituitary gland can also promote growth and regulate glucose metabolism. Neurohypophysis mainly transmits hypothalamus to regulate pituitary secretion function.

2) thyroid gland

The thyroid system of carp occurs at the bottom of gill cage, and is scattered in small pieces (small sacs) on the abdominal aorta, basal branchial bone and the ventral surface of sternal thyroid muscle in throat area.

3) Ultimate gill body

It is produced by epithelial cells located in the last branchial fissure between esophagus and venous sinus. The posterior branchial body can produce calcitonin, which can prevent the blood calcium content from being too high and inhibit the disintegration of bone tissue by osteoclasts.

Why is hippocampus a fish?

Hippocampus: The hippocampus is flat, without scales, and its trunk is surrounded by old wooden boards. It has a spine and fins, breathes by the gills, and lives in water all its life.

Characteristics of fish: You can get food and defend against enemies by swimming, and you can breathe in the water.

Fish often have scales on their bodies, breathe through their gills, and swim in harmony with their tails and fins.