Extant jawless fish are either lamprey or hagfish. Pteraspidomorphi is an extinct class of early jawless fish ancestral to jawed vertebrates. Lampreys form a superclass containing 38 known extant species of jawless fish. In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. Among jawless fishes, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. The only female vertebrates to lack oviducts are the jawless fishes. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms. It is present in all vertebrates above the jawless fish. This fauna was composed of conodonts, arthropods, possible jawless fishes and algae. Since then, opinions on the relations among jawless vertebrates have varied. Formerly, they could be divided into jawed and a jawless orders. Placement of this group among the jawless vertebrates is a matter of dispute. Some fish, both bony fish (Osteichthyes) and jawless fish (Agnatha), undergo metamorphosis. In jawless fishes, two subsets of lymphocytes use variable lymphocyte receptors (VLRs) for antigen binding.