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Message definition (Fish ) Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, notably [[Mormyridae|mormyrids]] and [[shark]]s, which have brains about as large for their body weight as birds and [[marsupial]]s. At the front of the brain are the [[olfactory bulb|olfactory lobes]], a pair of structures that receive and process signals from the [[nostril]]s via the two [[olfactory nerve]]s. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the [[telencephalon]], which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the [[diencephalon]]; it works with [[hormone]]s and [[homeostasis]]. The [[pineal body]] is just above the diencephalon; it detects light, maintains [[circadian]] rhythms, and controls color changes. The [[midbrain]] contains the two [[Midbrain#Corpora quadrigemina|optic lobes]]. These are very large in species that hunt by sight, such as [[rainbow trout]] and [[cichlid]]s. The [[metencephalon|hindbrain]] controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and [[lamprey]]s, but very large in [[mormyrid]]s, processing their [[electric fish|electrical sense]]. The brain stem or [[myelencephalon]] controls some muscles and body organs, and governs respiration and [[osmoregulation]].
魚の脳は体の大きさに比べて他の脊椎動物よりも小さく、同程度の大きさの鳥や哺乳類の脳の質量の15分の1程度である。しかし、比較的大きな脳をもつ魚も存在し、とりわけモルミルス科 やサメ は、体重に対する脳の大きさが鳥や有袋類 に匹敵する。脳の前方には嗅葉 があり、これは一対の構造で、二つの嗅神経 を通じて鼻孔 からの信号を受け取り処理する。ヤツメウナギやサメのように主に嗅覚で狩りを行う魚は非常に大きな嗅葉をもつ。その後方には大脳 があり、魚では主に嗅覚を処理する。これらの構造を合わせて前脳を形成する。前脳と中脳をつなぐのが間脳 であり、ホルモン や恒常性 に関与する。松果体 は間脳のすぐ上にあり、光を感知し、概日 リズムを維持し、体色変化を制御する。中脳 には二つの視葉 が含まれる。これらはニジマス やシクリッド のように視覚で狩りを行う種で非常に大きい。後脳 は遊泳と平衡を制御する。単葉の小脳は脳の最大部分であり、ヤツメウナギやヌタウナギでは小さいが、モルミルス科 では非常に大きく、彼らの電気感覚 を処理する。脳幹すなわち延髄 は一部の筋肉や体の器官を制御し、呼吸と浸透圧調節 を統御する。