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.
The body is generally spindle-shaped and divided into three parts: head, trunk and tail. The body surface has small shield scales, the body side has a lateral line, and the body length is about 1-20 meters. The bones of the whole body are cartilage, the skull and vertebrae are calcified, so there is a certain degree of hardness, and there is no bone suture in the skull. The tip of the head is flattened, slightly thicker near the pectoral fin, tapering backwards, the abdomen is relatively flat, the mouth is on the ventral surface of the head, the transverse fissure, and the upper and lower jaws have well-developed sharp teeth. The front of the mouth has 1 pair of nostrils. The gill fissures are located on both sides of the head, with 5-7 gill on each side, no gill caps, the gills are well spaced, and the gills are folded by epithelial tissue
shark
It forms a grid plate, so the shark (selachii) is also known as the plate gills (Elas-mobranchii). 1-2 dorsal fins, flattened tail, well-developed caudal fins, crooked tail, suitable for swimming in the middle and upper waters. 1 fin, some disappeared. 1 pair each of the pectoral and ventral fins; There is a cloaca hole between the ventral fins, and the inside of the male shark's ventral fin has a pair of finnipedes (maters), and the inside of the fins has a deep groove, and when mating, the two grooves merge into a tube that can transport semen to the female for in vivo fertilization. It is a carnivorous and ferocious fish (a more docile one that lives on the bottom of the ocean) and feeds on fish and crustaceans. There are spiral valves in the intestine. No swim bladders. Most are oviparous and a few are oviparous.
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