All are ectoparasites on some fishes. Mouth is circular and jawless. Head and brain poorly developed.
Eg. Petromyzon, Myxine.
Petromyzon(the lamprey) belongs to the section Agnatha of the sub-phylum-Vertebrata. They have long, greenish brown, cylindrical body with smooth scaleless, slimy skin, jawless mouth, etc. They lay eggs in freshwater but their ammocoete larvae(lower) after metamorphosis return to the ocean.