Jawed vertebrates or gnathostomes include fishes, amphibians, reptiles, birds and mammals, and all possess a well-developed brain enclosed within the cranium. The basic plan of the brain remains similar in all groups, being differentiated into forebrain, midbrain and hindbrain, but there are marked differences in size, complexity and functional specialization which reflect their mode of life. A generalized diagram of the vertebrate brain shows the olfactory lobes, cerebral hemispheres, diencephalon, optic lobes, cerebellum and medulla oblongata arranged in an antero-posterior sequence. In fishes, the brain is relatively simple and elongated. The olfactory lobes are large and prominent, especially in cartilaginous fishes, indicating a highly developed sense of smell. The cerebral hemispheres are small and smooth, lacking a true cortex. The diencephalon bears a well-developed pineal body. The midbrain is represented by large optic lobes, which are the most dominant part of the fish brain, correlating with the importance of vision. The cerebellum is moderately developed in active swimmers and