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External Brain Form and Cranial Nerves of the Megamouth Shark, Megachasma pelagios
Hironobu Ito, Masami Yoshimoto and Hiroaki Somiya
Vol. 1999, No. 1 (Feb. 5, 1999), pp. 210-213
Stable URL: http://www.jstor.org/stable/1447405
Page Count: 4
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The external brain form and cranial nerves of the megamouth shark, Megachasma pelagios, were studied morphologically. The brain of a mature female specimen, 5.44 m long and 1040 kg in weight, was examined. The brain is small, weighing 19.8 gm, and is positioned caudally in an extremely large cranial cavity. The length between the rostral end of the telencephalon and the caudal end of the medulla oblongata is 97 mm. The olfactory bulbs and the telencephalon are well developed, and the inferior lobe is rather small. The optic tectum is medium in size. The corpus cerebelli is proportionally small, whereas the auricle is large. The medulla oblongata is so widely everted that the visceral afferent, visceral efferent, and somatic efferent columns, as well as the sulci between them, are clearly visible in the rhomboid fossa. Terminal nerves were not found. The olfactory tracts and cranial nerves course for long distances within the expanded cranial cavity. Allometry of the brain size and the functional importance of the crainal nerves in this rare species are discussed in comparison with those of other cartilaginous fishes.