1. Shark: It belongs to cartilaginous fish, and its endoskeleton is completely composed of cartilage, which is often calcified, but there is no real bone tissue; The exoskeleton is underdeveloped or degenerated, and the body surface is often covered with scales. There are 5 ~ 7 gill holes on each side, which are open in vitro respectively. Don't swim (so sharks need to swim all the time or they will sink). The egg is large, rich in yolk, discoid and fertilized in vivo. Oviposition, viviparous or viviparous. Shark's fin is a good thing and is made into shark's fin. There are many kinds of sharks, which have always been regarded as the fiercest creatures, but this statement is unfair. Like whale sharks, they are the largest fish in the world and only feed on small marine animals. According to statistics, there are more than 380 species of sharks in the world, and only more than 30 species will take the initiative to attack humans, among which the great white shark is notorious. In addition, some sharks are cute and happy, like sideburns sharks, and their heads are like sloths in the ice age.

2, swordfish: also known as swordfish. It is named after its upper jaw extends forward like a sword. A flying fish belonging to the family Perciformes. Distributed in Indian Ocean, Atlantic Ocean and Pacific Ocean, the American coast in the west of Atlantic Ocean is the main producing area of swordfish. In China, swordfish is active in the vast open seas from the East China Sea of China, the Taiwan Province Strait to the South China Sea, and fishermen have caught swordfish in these waters. The upper jaw of swordfish is sharp and long, like a sword, stretching straight forward. Its body is prismatic, its back is dark brown and its abdomen is silver gray. It is 4-5 meters long, up to 6 meters long and weighs about 300 kilograms. It is a fierce big fish. Swordfish, though fierce, are timid and tend to avoid other large fish. Swordfish live in the upper layer of tropical and subtropical oceans and feed on fish and cephalopods, such as shrimp and squid. Every year 1 month is the breeding season. Swordfish is one of the fastest swimming fish, because it has strong swimming ability and extremely fast speed, and can swim into the sea every hour. It usually swims on the water, sometimes showing its dorsal fin and sometimes jumping out of the water. This kind of fish doesn't swim in groups, and each fish keeps at least a distance of less than meters. The arrow-shaped long kiss is the main weapon for swordfish to attack and prey. It flies out of the sea with great explosive force and often rushes out of the sea to attack large whales and fish with sword-shaped jaws. It is said that swordfish also attacked ships, causing them to sink. Swordfish is also an economic fish with high value. Its meat is bright red and its liver is rich in vitamins. Fishermen in Taiwan Province Province of China caught them with darts, and fishermen in Xisha Islands of the South China Sea caught them with longlines.

3. Whales: Most species live in the ocean, and only a few species inhabit the freshwater environment. Their bodies are very similar to fish, streamlined and suitable for swimming, so they are commonly known as whales, but this similarity is only a convergence phenomenon in biological evolution. Cetaceans belong to mammals because they have the characteristics of viviparous, lactation, constant temperature and breathing with lungs, which are completely different from fish. The heartbeat of whales is usually 9 ~ 10 times per minute.

Cetaceans are characterized by constant body temperature, about 35.4℃. Bare skin, no body hair, only a few bristles on the nose and mouth, no sweat glands and sebaceous glands. The fat under the skin is very thick, which can keep the body temperature and reduce the proportion of the body in water. The skull is developed, but the skull is smaller, the face is larger, the frontal bone and maxilla are obviously prolonged, forming a long snout. The neck is not obvious, the cervical vertebra is healed, and the head is directly connected with the trunk. The forelimbs are fin-shaped, the toes are not separated, there are no claws, and the elbow and wrist joints cannot move flexibly, which is suitable for swimming in the water. The hind limbs are degenerated, but there are still pelvic and femoral residues, and residual bone fragments can be seen. The tail degenerates into fins, and the skin at the end expands horizontally from left to right, forming a pair of large tail leaves without bone support. The spine gradually tapers in the long and narrow caudal trunk and finally disappears before entering the caudal fin. Unlike fish, the caudal fin can swing up and down and is the main organ for swimming. Some species also have dorsal fins to balance their bodies. Their bones have spongy tissue, and there is more fat in the body cavity, which can increase the volume of the body, reduce the proportion of the body and increase buoyancy.

Their eyes are small, without lacrimal glands and transient membranes, and their eyesight is poor. There is no outer ear shell, and the external auditory canal is very thin, but the hearing is very sensitive. It can feel ultrasonic waves, find food by echolocation, contact companions or avoid enemies. There are 1 ~ 2 external nostrils, located on the top of the head, commonly known as spray holes. Generally, the closer the nostril is, the higher the degree of evolution. Breathing with lungs, there is a lung on the left and right, and there are many capillaries in it, which are elastic and can help the circulation of oxygen and adapt to the gas exchange on the water. You need to surface for air every once in a while, and you can also dive for a long time. There are 10 ~ 20 pairs of ribs. The stomach is divided into four chambers. Most kidneys are tumor-like. Male testicles are located in the abdominal cavity. Female animals give birth and breastfeed in water. The uterus has two horns and a pair of breasts, which are located at the cleavage on both sides of the reproductive fissure, and the nipples are slender. Milk is rich in calcium, phosphorus and a lot of fat. Young whales have teeth in the embryonic period, but whales' teeth are replaced by beards at birth, and toothed whales' teeth are preserved for life.