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| regnum = Animalia | phylum = Chordata | classis = Chondrichthyes | subclassis = Elasmobranchii | subclassis_authority = Bonaparte, 1838 }} Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish that includes the sharks (Selachii) and the rays, skates, and sawfish (Batoidea). "They resemble the true fishes in external form, but differ from them so widely in structure that they are placed in a class by themselves."〔Jordon, "Guide to the Study of Fishes, Vol. I. pp506-511〕 ==Description== Elasmobranchii is one of the two subclasses of cartilaginous fish in the class Chondrichthyes, the other being Holocephali (chimaeras). Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. Extant elasmobranchs exhibit several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support. In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.〔Wilga, C. D. 2005. Morphology and evolution of the jaw suspension in lamniform sharks. ''Journal of Morphology'', 265, 102-119.〕〔Wilga, C. D., Motta, P. J. & Sanford, C. P. 2007. Evolution and ecology of feeding in elasmobranchs. ''Integrative and Comparative Biology'', 47, 55-69.〕〔Wilga, C. A. D. 2008. Evolutionary divergence in the feeding mechanism of fishes. ''Acta Geologica Polonica'', 58, 113-120.〕 The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters. Many fish maintain buoyancy with swim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil.〔Oguri, M (1990) ( "A review of selected physiological characteristics unique to elasmobranchs" ) In: ''Elasmobranchs as living resources: advances in the biology, ecology, systematics and the status of the fisheries'', eds. J. H. L. Pratt, S. H. Gruber and T. Taniuchi, US Department of Commerce, NOAA technical report NMFS 90, pp.49–54.〕 This stored oil may also function as a nutrient when food is scarce.〔Hoenig, J.M. and Gruber, S.H. (1990) ("Life-history patterns in the elasmobranchs: implications for fisheries management" ) In: ''Elasmobranchs as living resources: advances in the biology, ecology, systematics and the status of the fisheries'', eds. J. H. L. Pratt, S. H. Gruber and T. Taniuchi, US Department of Commerce, NOAA technical report NMFS 90, pp.1–16.〕 〔Bone Q and Roberts BL (1969) ("The density of elasmobranchs" ) ''Journal of the Marine Biological Association'', 49: 913–937.〕 Deep sea sharks are usually targeted for their oil, because the livers of these species can weigh up to 20% of their total weight.〔Vannuccini, Stefania (2002) (Shark liver oil products ) In: ''Shark Utilization, Marketing and Trade'', Fisheries Technical paper 389, FAO, Rome. ISBN 92-5-104361-2.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「elasmobranchii」の詳細全文を読む スポンサード リンク
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