A picture of two silver lampreys, Ichthyomyzon unicuspis, some of the most primitive existing vertebrates. Like the earliest vertebrates, they are long and eel-like. True eels, however, are rather distantly related to lampreys. Unlike eels and other fishes, lampreys do not have jaws or pectoral fins. Instead, they have a sucker mouth, visible in the lower right, which they use to feed on other fishes. The lamprey nervous system is being used as a model to develop prosthetic devices for people with spinal cord injuries.