The behemoth in the sea-Cetaceans
The Latin scientific name of cetaceans comes from the Greek word "sea monster", which shows the ancient people's reverence for this giant living in the ocean. In fact, the size of cetaceans varies greatly, from about 1 meter to over 30 meters. Most of them live in the ocean, and only a few live in fresh water 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.
Cetaceans are characterized by constant body temperature, about 35.5℃. 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.
The ancestors of cetaceans used to walk on all fours on land, probably herbivores or carnivores mainly living in coastal areas. Later, they were attracted by fish and other foods in the water. After a long time, they returned to the ocean from land and gradually adapted to marine life. The earliest whales appeared in about 55 million years? /FONT & gt； 36 million years ago, primitive whales in the middle Eocene, primitive whales in the late Eocene, etc. Compared with the existing whales, their skulls are smaller, and their nostrils are located in front of their heads, but they have not moved to the top of their heads. Their teeth are similar to those of ancient insectivores and carnivores, with 44 or fewer teeth. Their teeth are also very similar in shape and skull, with only a few differences, but they have adapted to living in seawater.
Baleen whales are huge, and the smallest species is more than 6 meters long. There are no teeth in the mouth, and only degenerated teeth can be seen during embryonic development, but there are 150-400 comb-like horny whiskers on the maxilla from the palate to the left and right sides of the pharynx. The color, shape and quantity of beards vary from species to species, which is one of the important basis for classification. There are two external nostrils, located on the top of the head, which can eject two water columns when breathing. The skull is extremely large, and some species can reach 1/3 of the body length, which is symmetrical. Healing or separation of cervical vertebrae. The sternum is small, only 1-2 pairs of ribs are connected with the sternum, and the thorax is incomplete. No collarbone. Flippers usually have four fingers. There is a cecum in the digestive tract. They mainly feed on small crustaceans such as krill, and some species also eat small groups of fish, as well as benthic fish and shellfish. There are three families of fin whales in the world, including right whale family, gray whale family and fin whale family, about 6 genera 1 1 species.
The shape of toothed whales varies greatly, with the smallest species being only about 1 m and the largest being more than 20 meters. There are conical teeth in the mouth, but the shape and number of different kinds of teeth are also very different. The least is 1 single tooth, the most is dozens, and some are hidden in gums, so it is also one of the important bases for classification. There are only 1 external nostril, so only one stream of water can be ejected when breathing. The skull is asymmetrical left and right. There are five fingers on the flippers. The sternum is bigger. No collarbone. No cecum. They mainly feed on squid and fish, and some can prey on large animals, such as seabirds, seals and other whales. There are 8 families, about 34 genera and 72 species of toothed whales in the world, including puffer fish, ray family, swordfish, narwhalidae, dolphin family, dolphin family and hairtail family.
Whales have been killed by humans since ancient times because of their great economic value, but in the past, due to the backward killing methods, the amount of whales killed was small, which was not enough to affect the number of whales. In modern times, people used ships and artillery to kill whales, which greatly increased their lethality, resulting in a sharp decline in the number of whales and many species on the verge of extinction.
Nowadays, because there are strict restrictions on the number of whales caught each year in the world, whaling is no longer the main concern of scientists who monitor the number and living conditions of whales. However, worldwide, unprecedented marine pollution caused by human economic development poses a great threat to whales, far greater than killing. In addition, the rapid development of industrial fishing has greatly affected the food sources of whales and other marine mammals, which is another major factor affecting their survival. In the past, people always imagined the ocean as an endless free world and a symbol of being able to take risks. Therefore, while plundering marine resources unscrupulously, the shipping industry not only produces a lot of marine noise and discharges a lot of ballast water every year, but also dumps a lot of garbage into the ocean. About 6,543,800 kinds of chemicals also reach the ocean through the discharge of sewage and air. Refractory organic chlorides increase the content of marine organic matter, which makes marine pollution more and more serious and brings serious problems.
The research results of modern science show that the ocean is a giant container for storing waste gas carbon dioxide, and the cold and hot ocean current system has a great influence on the earth's climate. There are many unknown animals and plants that are beneficial to human beings, including many potential sources of food and medicine for human beings, which are of great value. The increasingly serious marine pollution will destroy these precious resources. Due to the destruction of the ozone layer, the population density of krill living in the waters around Antarctica has dropped sharply, and the number of krill per 1000 cubic meters of seawater has dropped sharply from 1982 to 177.8 before 1983, and then to 46538+0985. Krill is a crustacean in the ocean and an important bait for marine mammals such as marine fish and whales. It plays an extremely important role in the marine biological chain, and has attracted much attention as the largest protein resource left over from the earth. Because cetaceans and other marine mammals are located at the end of the food chain, marine pollution, especially heavy metal pollution and other turbulent substances seriously weaken their immune systems, making them vulnerable to viruses and bacteria. Pollution will also prevent women from getting pregnant or having an abortion, thus greatly reducing their reproductive rate.
China is rich in cetacean resources. Up to now, 38 species belonging to 26 genera and 9 families have been found in China's sea areas, including the blue whale with a body length of more than 30 meters, and the finless porpoise with a body length of only about 1 meter, especially the freshwater cetacean baiji, which is a wonderful flower among cetaceans. Except baiji and Chinese white dolphin, all cetaceans in China are classified as second-class protected animals.
Whales are mammals that live in water. It has the same physiological characteristics as terrestrial mammals, such as breathing with lungs and viviparous. And has some special physiological structures that have evolved to adapt to the aquatic environment. Whales belong to the animal kingdom, Chordata, Mammalia and Cetacea.
There are two suborders under Cetacea, namely Blaleenwhales and Megaodon. The classification of these two groups is mainly determined by their different feeding methods. The main morphological feature of the suborder baleen whales is that they have no teeth, but there are great baleen whales, which can be used to screen plankton, so they are filter feeders. The main feature of Toothed Whale suborder is that it has teeth and is predatory, and the number and arrangement of teeth will be different from the direction of predation. There are about 79 species of 13 families in the world.
Although the whale has the word fish, it is actually not a fish, but a mammal. It has many characteristics very different from fish. For example, the average fish swings its tail fin left and right to move forward, while the whale swings its tail fin up and down. They use flippers at the front end to keep balance and control the direction of force. Some whales have fins at the top of their backs to keep their bodies upright!
Whales are social animals. They usually live in groups in the sea, but when whales breathe, they need to swim to the surface. At this time, the whale breathes through the blowhole on its head, and when it exhales, the water in the air will condense and form the familiar fountain shape. Experts can even tell the species of whales from the height, width and angle of water spray! There are many kinds of whales, which can be roughly divided into toothed whales and baleen whales.
Whales have a very thick layer of fat under their skin, which is also commonly known as whale oil. It can keep whales warm and store energy for a rainy day. Because whales have many special structures in their bodies, they can breathe in the water for a long time and slow down their heartbeat. Therefore, when it sinks to the bottom of the sea, it will take a long time to surface again. In addition to the oxygen storage structure, when a certain part of the body needs a lot of blood supply, there will be a special function of centralized supply in the body!
The water column ejected by baleen whales is high and thin, while the water column ejected by toothed whales is thick and short. Some farmers can even judge the species of whales according to the water column!