Before dinosaurs appeared, sharks lived on the earth for 300 million years and remained unchanged for more than 500 million years. Sharks, called squid, squid and sand fish in ancient times, are behemoths in the ocean, so they are called "wolves in the sea".
Traditionally, shark cartilage (shark fin) has a high content of protein, but this is wrong. Eggs contain more protein than shark fins. In addition, research shows that sharks can easily accumulate mercury, and shark fin contains a certain amount of neurotoxin, which is potentially harmful to human beings. Due to decades of large-scale killing, sharks are in danger of extinction.
The body is generally spindle-shaped and is divided into three parts: head, trunk and tail. There are small scales on the body surface and lateral lines on the body side, and the body length is about 1-20 meters. The bones of the whole body are cartilage, and the skull and spine are calcified, so there is a certain hardness, and there is no seam in the skull. The head is flat, slightly thicker near the pectoral fin, gradually tapering backward, the abdomen is flat, the mouth on the ventral surface of the head is transverse, and the upper and lower jaws have developed sharp teeth.
Extended data:
The types of sharks are:
One or six gill sharks
There are 2 families, 4 genera and about 5 species, including 6 gill sharks and wrinkled gill sharks. In China, there are only 4 species belonging to 3 genera 1 family. Widely distributed in tropical and subtropical waters of the Pacific Ocean and Atlantic Ocean. Original structure. There are 6 ~ 7 gill holes. There is no blink or blink in the eye. There are water spray holes.
1 dorsal fin, without spines, posterior position, with gluteal fin; The axial bone of the pectoral fin extends to the anterior edge of the fin, and there are no radial fins in the anterior fin cartilage. The vertebral body is not completely divided, but the vertebral body is calcified, and the spinal cord is partially or not contracted. Kissing cartilage 1. Viviparous
Second, the order of rats and sharks
There are 5 families, including conodont shark, sword shark, mako shark (Sharidae), basking shark and long-tailed shark. There are 4 families, 6 genera and 8 species in China. Five pairs of gill holes. 2 dorsal fins, spineless; There are anal fins. There is no blink or blink in the eye. The vertebral body has radial calcified areas, and 4 non-calcified areas have no calcified spokes. Maxillolingual joint type, the upper jaw is connected to the skull only by ligament.
Third, tiger sharks.
Only 1 family, 1 genus, 8 species. Distributed in tropical and temperate waters of the Pacific Ocean and Indian Ocean. There are two kinds of tiger sharks in China: the broad-banded tiger shark and the narrow-banded tiger shark. Small and medium-sized sharks, up to 1.5m long. The body is thick and short, and the head is nearly square. There are various stripes or spots on the body. The kiss is short and blunt, with small eyes, oval shape and upper side position. The nose has a nasal groove. The mouth is flat and horizontal, and the upper and lower lips are pleated.
The upper and lower teeth are of the same type, the front and rear teeth of each jaw are irregular, the front teeth are pointed, and there are 3 ~ 5 teeth; The posterior teeth are flat and molar-shaped. The water spray hole is small and located behind the eyes. There are five gill holes. Two dorsal fins, each with 1 spines, can defend the enemy; There are anal fins; The caudal fin is short and wide and broom-shaped. The pectoral fin is very wide. Habitat on the seabed, feeding on shellfish and crustaceans. Two eggs are laid each time, and the eggs have a conical horny sac with filaments at the end to be fixed on the attachment.
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