You can tell at a glance whether it is a class without a chin or a class with a round mouth. Either lamprey or blind eel. The dorsal fin can't see clearly, and it seems that there are no tentacles. That is lamprey.

See how many gill holes there are. The lamprey has no lateral fins, only odd fins. There are 1 eyes without eyelids on the head side; There are 7 gill holes on both sides of the back of the eye; There are lateral lines composed of sensory fossa on both sides of the body; Tail fin is the most primitive protocaudate in vertebrates;

Blind eels and lamprey are blood-sucking animals. By the way, see if they have eyes, so as to distinguish blind eels from blind eels.