[The brain of the Agnatha]

The brain of the Agnatha, especially of the hagfish, is discussed from the viewpoint of the telencephalon and the olfactory, visual, trigeminal, and vestibulolateral sensory systems. Myxiniformes and Petromyzontiformes, which were an independent group in the Ordovician, are thought to represent some parallels of ancestral vertebrate brains. It is interesting to study the brain of the Agnatha to investigate the process of the evolution of the vertebrate brain. In comparison with the lamprey, the hagfish has subcutaneous eyes under an unpigmented patch of skin, 4 paired tentacles at the rostral tip of the head, one semicircular canal on each side, a regressed ventricular system, a "primordium hippocampi" of unresolved homology in the telencephalon, no pineal body, fusion of the habenula of both sides, a tectum opticum with unclear laminations, no macroscopical cerebellum, and optic decussation within the hypothalamus. The arrangements of the trigeminal subnuclei of the descending tract are different from each other, unlike those of other gnathosomata. In the hagfish, the distribution of fibers carrying input of the vestibulum and lateral line system within the area vestibulo-lateralis differs from that of the lamprey, which resembles the teleosts and cartilaginous fish in this respect. These differences may be caused by an independent origin within the Agnatha. The basic organization of the brain of the Agnatha, such as the prosencephalon, mesencephalon, and rhombencephalon are common to the gnathostomata, although some variations in development are included in the sensory centers and higher centers of information processing. The sensory neurons within the brain are found in the medulla oblongata in the Agnatha, whereas in the gnathostomata the mesencephalic nucleus of the trigeminal nerve has been formed in connection with the development of the mandibula.