In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. Both lampreys and hagfish possess a fully developed medulla oblongata. Together hagfish and lampreys are the sister group to vertebrates. Hagfish and lamprey kidneys are unusually simple. The organ is active in adult forms of some primitive fish, like lampreys or hagfish. Smaller pieces became lamprey and hagfish. It is now believed that the hagfish neuroanatomy is similar to that of lampreys. Their classification as agnathans places hagfish as elementary vertebrates in between invertebrates and gnathostomes. The most important exception is a primitive vertebrate, the hagfish. Other predators of hagfish are varieties of birds or mammals. However, discussion has long occurred in scientific literature about whether the hagfish were even invertebrate. They resemble primitive bone marrow in hagfish. The inshore hagfish, found in the Northwest Pacific, is eaten in Japan. The single canal found in hagfish appears to be secondarily derived. The first diagram of the hagfish endoskeleton was made by Frederick Cole in 1905.