美味偏执狂
Out of all the species walking, flying, slithering or swimming, there aren't many who have been around as long, survived as well, or come in so many shapes and kinds as the shark. The earliest evidences of sharks are isolated spines, teeth and scales that appeared about 430 million years ago in the Silurian Period, known as the "Age of Fishes". Sharks have a sleek, streamlined design which helps them swim without using up a lot of energy.They certainly need to conserve their energy because they never really sleep and most of them never stop swimming. Some sharks are fierce predators, and would be happy to eat you if they encountered you. Almost any shark six feet or longer is a potential danger, but three species have been identified repeatedly in attacks: the Great White Shark, the Tiger Shark and the Bull Shark . All three live world wide, reach large sizes and eat large prey such as marine mammals or sea turtles. But most sharks never grow longer than five feet and never even see anyone with legs and arms anyway. People kill thousands more sharks every year than sharks kill people. Sharks take about as long to mature as we do. Some of them become adults in their teens. A mother shark carries her babies inside her body while they develop, sometimes for more than a year. Even so, some sharks are born inside an egg which they have to crack open. They spend early portions of their lives in nursery grounds. Some of the advantages sharks have over people is that they keep growing new teeth, they don't have breakable bones, and they are not prone to get cancer. Sometimes sharks are referred to as swimming computers because of the six senses which they possess: vision, hearing, vibration, smell, taste and electro-perception.参考资料：
大头的陈小晶
were interested in discovering just how the doomed hero chose to spend his last days
幸福顺延
鲨鱼属于脊椎动物门软骨纲(Chondrichthyes)板鳃亚纲，海生，少数种类进入淡水，为一群游速快的中大型海洋鱼类。
Sharks are vertebrates steel door cartilage (Chondrichthyes) elasmobranchii, Marine, a few species enter fresh water, as a group of large Marine fish speed is fast.
鲨内骨骼完全由软骨组成，常钙化，但无任何真骨头组织，外骨骼不很发达或退化，身体常被皮齿(质鳞)，牙多样化，具有硬肌，但膜骨则永不存在，脑颅无缝。
Shark endoskeleton completely composed of cartilage, often calcified, but without any true bone tissue, degraded or exoskeletons is not very rich, the body is often skin teeth (scale), tooth diversification, with hard muscle, but never exist, membrane bone temple seamless.
上额由腭方软骨组成，下额由梅氏软骨组成。最新研究表明，鲨体内含有多种生物活性成分，如抗癌因子、角鲨烯、黏多糖等等。
On the forehead is composed of palatal side cartilage, the forehead is composed of Mr Medvedev cartilage.Sharks, according to new research in the body contains a variety of biological active ingredients, such as anti-cancer agent, squalene, mucopolysaccharides and so on.
扩展资料
鲨鱼的起源不详，但其地质学纪录可追溯到泥盆纪(4.08亿～3.6亿年前）。鲨鱼化石在中泥盆世出现，在石炭纪（3.6亿～2.86亿年前）成为占优势的脊椎动物。现代鲨鱼出现于侏罗纪早期（2.08亿～1.87亿年前），到白垩纪（1.44亿～6,640万年前）已经发展成现存的各科。
总之，除觅食和游泳机制的改进外，进化对鲨鱼的形态改变甚微。无论对化石种和现存种类来说，牙齿在鲨鱼种类鉴别方面均有很重要作用。
参考资料来源：百度百科-鲨鱼
李出于蓝
sharksareatypeoffish.therearehundredsofkindsofsharks.mostareabouttwometerslong.thedogfishshark,however,islessthantwentycentimetersinlength.thebiggestwhalesharkcangrowtotwentymetersinlength.sharksdonothavebones.theskeletonofasharkismadeofcartilage.humannosesandearsarealsomadeofcartilage.asharkhasanextremelygoodsenseofsmell.itcanfindsmallamountsofsubstancesinwater,suchasblood,bodyliquidsandchemicalsproducedbyanimals.sharksalsosenseelectricalandmagneticenergylinkedtonervesandmusclesoflivinganimals.这个好像超了100字了，你可以删点儿内容
桑珠欢穆
What is a Shark?Sharks are a type of fish.Sharks are amazing fish that have been around since long before the dinosaurs existed. They live in waters all over the world, in every ocean, and even in some rivers and lakes. Unlike bony fish, sharks have no bones; their skeleton is made of cartilage, which is a tough, fibrous substance, not nearly as hard as bone. Sharks also have no swim bladder (unlike bony fish). ________________________________________MAJOR DIFFERENCES BETWEEN SHARKS AND BONY FISHATTRIBUTE SHARKS BONY FISHSkeleton Cartilage only Bones and cartilageSwimming Can only swim forward. Can swim forwards and backwardsBuoyancy (floating) Large oily liver Gas-filled swim bladderGills Gill slits but no gill cover Covered gill slitsReproduction Eggs fertilized in female's body. Eggs usually fertilized in the water.Skin Rough, sandpaper-like placoid scales Slippery, overlapping scalesSIZEA scuba diver above a Whale shark. The Whale shark is the largest fish in the world; the basking shark is the second largest fish.There are many different species of sharks that range in size from the size of a person's hand to bigger than a bus. Fully-grown sharks range in size from 7 inches (18 cm) long (the Spined Pygmy shark), up to 50 feet (15 m) long (the Whale shark). Most sharks are intermediate in size, and are about the same size as people, 5-7 feet (1.5-2.1 m) long. Half of the 368 shark species are under 39 inches (1 m) long. ________________________________________BODY SHAPESSharks have a variety of body shapes. Most sharks have streamlined, torpedo-shaped bodies that glide easily through the water. Some bottom-dwelling sharks (e.g. the angelshark) have flattened bodies that allow them to hide in the sand of the ocean bed. Some sharks have an elongated body shape (e.g., cookiecutter sharks and wobbegongs). Sawsharks have elongated snouts, thresher sharks have a tremendously elongated upper tail fin which they use to stun prey, and hammerheads have extraordinarily wide heads. The goblin shark has a large, pointed protuberance on its head; its purpose is unknown. ________________________________________VARIETIES OF SHARKSThere are about 368 different species of sharks, which are divided into 30 families. These different families of sharks are very different in the way they look, live, and eat. They have different shapes, sizes, color, fins, teeth, habitat, diet, personality, method of reproduction, and other attributes. Some types of shark are very rare (like the great white shark and the megamouth) and some are quite common (like the dogfish shark and bull shark). Sharks belong to the group of cartilagenous fish, the Elasmobranchii, that includes the sharks, rays, and skates.There are over 350 different species of sharks. The spiny dogfish shark is the most common shark.
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