In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. Both lampreys and hagfish possess a fully developed medulla oblongata. Lampreys and hagfish do not have gill slits as such. The organ is active in adult forms of some primitive fish, like lampreys or hagfish. The primitive lampreys and hagfish, however, have a simpler system. It is now believed that the hagfish neuroanatomy is similar to that of lampreys. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes. The only vertebrates lacking a spleen are the lampreys and hagfishes. The only vertebrates lacking a spleen are the lampreys and hagfishes (the Cyclostomata). Some fish such as lampreys, trout and salmon go upstream to spawn. It has been found in all vertebrates except lampreys and hagfishes. Some fish, like sharks and lampreys, possess multiple gill openings. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks. River fish of note include trout, Atlantic salmon, pikes, shades and lampreys. There are about 38 known extant species of lampreys and five known extinct species.