<P> Fish brains are divided into several regions . At the front are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves . The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish . Behind the olfactory lobes is the two - lobed telencephalon, the structural equivalent to the cerebrum in higher vertebrates . In fish the telencephalon is concerned mostly with olfaction . Together these structures form the forebrain . </P> <P> Connecting the forebrain to the midbrain is the diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with hormones and homeostasis . The pineal body lies just above the diencephalon . This structure detects light, maintains circadian rhythms, and controls color changes . </P> <P> The midbrain (or mesencephalon) contains the two optic lobes . These are very large in species that hunt by sight, such as rainbow trout and cichlids . </P> <P> The hindbrain (or metencephalon) is particularly involved in swimming and balance . The cerebellum is a single - lobed structure that is typically the biggest part of the brain . Hagfish and lampreys have relatively small cerebellae, while the mormyrid cerebellum is massive and apparently involved in their electrical sense . </P>

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