Sharks belong to the chondrichthyes suborder Chondrichthyes , marine , a small species that enter fresh water and are a group of medium to large marine fish that swim fast.
There are many types of sharks, and at least 440 shark species can be distinguished in the world's oceans. The Sharks of the World series chronicles more than 500 books.
The subordinates of the general order of lateral holes are divided into 8 orders and 25 families:
Six-gill shark
1． Hexanchiformes: There are about 5 species in 2 families and 4 genera, including Heaxanchidae of the family Heaxanchidae and chlamydoselachidae of the family Tilidae. In China, there are only 3 genera and 4 species in the heaxanchidae family Heaxanchidae1 family. It is widely distributed in the tropical and subtropical waters of the Pacific and Atlantic Oceans. The structure is primitive. 6 to 7 gill holes. There is no transient membrane or transient fold in the eye. There are water spray holes. 1 dorsal fin, no hard spines, posterior position, fin; the central axis bone of the pectoral fin extends to the anterior edge of the fin, and the cartilage of the front fin is unraveled. The spinal segmentation is incomplete, but the vertebral body is more calcified, and the spinal cord is partial or not constricted. Kiss 1 cartilage. ovoviviparous.
2． Lamniformes: There are 5 families, including Carchariidae of the cone-toothed shark family, Scapanorhynchidae of the sword-snouted shark family, Isuridae of the mackerel family (Lambidae of the porphyllaceae), Cetorhinidae of the basking shark family, and Alopiidae of the long-tailed shark family. There are 4 families and 6 genera and 8 species in China. 5 pairs of gill holes. 2 dorsal fins, no spines, fins. There is no transient membrane or transient fold in the eye. The vertebral body has radial calcification areas, and 4 non-calcified areas are free of calcified spoke invasion. The mandibular tongue is attached, and the upper jaw is attached only to the skull by ligaments.
3． Heterodontiformes: 8 species in only 1 family and 1 genus. It is found in tropical and temperate seas of the Pacific and Indian Oceans. Our domestic wide-striped tiger shark Heterodontus japonicus and narrow-striped tiger shark Heterodontus zebra are two kinds. Small and medium-sized sharks, up to 1.5 meters long. The body is large and short, and the head height is nearly square. The body has various stripes or spots. The snout is short and blunt, the eyes are small, oval, and the upper lateral position. The nostrils have a nasal groove. The mouth is flat and transverse, and the upper and lower lip folds are well developed. The upper and lower teeth are of the same type, the anterior and posterior teeth of each jaw are at different shapes, the anterior teeth are finely pointed, and the 3 to 5 teeth are headed; the posterior teeth are flattened and molar-like. The spray hole is small and located behind the eye. 5 gill holes. 2 dorsal fins, each with 1 hard spine, can withstand predators, with fin, and the tail fin is wide and short, broom-shaped. The pectoral fins are wide. It inhabits the bottom layer and eats shellfish and crustaceans. Each spawn is laid 2 eggs with a conical keratinous sac with a filament at the end that secures to the attachment.
4． Squaliformes: There are more than 200 species in 4 suborders, 7 families, and 49 genera, including the suborder Scyliorhinoidei, the suborder Triakoidei, the suborder Carcharhinoidei, and the hammerhead shark suborder Sphyrnidae. There are about 60 species in 4 suborders, 5 families and 23 genera in China. 2 dorsal fins, no hard spines, fins. 5 gill holes. Jaw-tongue joint type. Kiss 3 cartilages. The eye has transient folds or transient membranes. The vertebral body has radial calcification areas, and 4 non-calcified areas have calcified spokes invading. The spiral lobes of the intestine are spiral or scroll-shaped.
5． Squaliformes: There are about 87 species in 3 families and 21 genera, including Squalidae of the family Squalidae, Dalatiidae of the family Oleidae, and Echinorhimidae of the spiny shark family. The dorsal fins are 2, usually each with a spine; the fin disappears. There are 5 gill holes, located in the low front of the pectoral fin base. Lateral or superior ocular position. It is mainly distributed in the warm, cold water or deep sea areas of the world.
6． Sawsharks (Pristiophoriformes): 5 species in 2 genera in only 1 family. The body length can reach 4 meters. The snout is long, sword-like protruding, with serrated edges. The ventral surface has 1 pair of leather whiskers in front of the nostrils. The head is quite flat. Superior lateral position of the eye. The water spray hole is large and is located behind the eye. The nostrils are round and far from the mouth. The teeth are small and pointed, and many lines are used. 5 to 6 gill holes. 2 dorsal fins, no hard spines; no fins. Perched on the ground floor, the kiss saw is a weapon of self-defense. Crustaceans, worms and small fish. Excellent meat quality. Sawshark genus Pristiophorus, 5 gill holes, a total of 4 species. In China, there is only 1 species of Japanese saw shark Pristiophorus japonicus, which is distributed in the Yellow Sea and the East China Sea, and occasionally in the South China Sea and the coasts of Korea and Japan. The six-gill sawfish genus Pliotrema, with 6 gill holes and only 1 species, is distributed on the southeast coast of southern Africa.
Flat shark
7． Squatiniformes: About 13 species in 1 family and 1 genus alone. Flattened body; short and wide snout; broad pectoral fin and extending to the side of the head such as robe sleeves, formerly known as sleeve fish, commonly known in the West as angel fish or monk fish; upper eye; broad mouth, sub-anterior position; upper teeth, jaw homotype, slender single-tooth head type; anterior nostril position; gill hole 5, wide, extended to the ventral surface; dorsal fin 2, no hard spines. It is widely distributed in the Tropical and Temperate Atlantic, Mediterranean and Pacific Oceans, and is also found on the eastern coast of southern Africa. There are 4 species in our country. It is a secondary economic fish in the Yellow Sea and the East China Sea, and has a large production in the Yellow Sea, and is one of the fishing objects. The East China Sea produces less and is a bottom-dragging net and fishing class.
8. Orectolobiformes: There are 7 families: Brachaeluridae of the family Longhair, Ginglymostomatidae of the family Ginglymostomatidae, Hemiscylliidae of the family Lyceumaceae, Orectolobidae of the family Balsylobidae, Family Parascyllidae of the family Balscalidae, Family Rhinocodontidae of the cetaceae, and Stegostomatidae of the leopard shark family. There are 3 families, 5 genera and 10 species in China. 5 pairs of gill holes. 2 dorsal fins, spineless, fins. There is no transient membrane or transient fold in the eye. The vertebral body has radial calcification areas, and 4 non-calcified areas are free of calcified spoke invasion. The maxillary type is joined, and the upper jaw is attached only to the skull by ligaments. Including the world's largest fish.
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