Terry catches an animal from the Class Osteichthys from the ocean and notices that, attached to its flank, there is an equally long, snakelike organism. The Osteightyhys looks frail and sickly, almost dead. The attached organism has no external segmentation, no scales. a round mouth surrounded by a sucker and two small eyes. Terry thinks it might be a marine leech, a hagfish, or a lamprey. Which feature excludes it from possibly being a leech?