Introduction:
Hydra is well known for its round body, generally about 100 ~ 300 mm long, and it can reach more than 630 mm Hydra mostly lives in the sea, but it can also be seen at the intersection of fresh water and sea water. They blow up water and air in the water, make the sediment fly, and then prey on the creatures hidden in the sand. Because teeth and jaws are hard, even extremely hard shells can be crushed.
It is oval, with a blunt front end and a tapered tail. The kiss is short, round and blunt; Small mouth, end position, transverse crack. There are two plate-shaped incisors in the upper and lower jaws, and the middle seam is obvious. The lips are developed, and both ends of the lower lip bend upward on the outer side of the upper lip. Small eyes, small gill holes, an arc crack, located in front of the pectoral fin, densely covered with small thorns. The dorsal fin is very backward, opposite to the gluteal fin; No ventral fins; The back end of the caudal fin is flat. The back of the body is grayish brown, the side is slightly yellowish brown, and the ventral surface is white; The marks on the back and side of the body vary from species to species.