|
The anamniotes are an informal group comprising the fishes and the amphibians, the so-called "lower vertebrates", which lay their eggs in water. They are distinguished from the amniotes, the "higher vertebrates" (reptiles, birds and mammals), which lay their eggs on land or retain the fertilized egg within the mother. The name refers to the amnion, an embryonic membrane produced during the fetal development of amniotes which serves to transport oxygen into the egg and expel carbon dioxide. As the name suggests, anamniotes do not develop an amnion during fetal life and are able to exchange oxygen, carbon dioxide and waste metabolites with the surrounding water.〔''Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time''. 4th edition. John Wiley & Sons, Inc, New York - ISBN 978-0-471-38461-8.〕 The water helps in the diffusion of waste products (particularly ammonia), allowing the embryo to complete embryonic development without being poisoned by their own waste products.〔Carroll R.L. (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) ''Origins of the higher groups of tetrapods — controversy and consensus''. Ithaca: Cornell University Press, pp 331-353.〕 During their development, all anamniote classes pass through a stage which resembles fish, thus indicating their close physiological relationship. ==Anamniote traits== The group is characterized by retaining the primitive vertebrate condition in several traits:〔Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985)〕〔Nicholson, H.A. (1880): ''Manual of Zoology'', Blackwood And Sons. (Original text )〕 * Permeable skin allowing diffusion of water and gases directly through the skin. * Presence at some period of life of gills * Absence of an amnion * Absence or rudimentary condition of the allantois * Nucleated red blood cells. 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「anamniotes」の詳細全文を読む スポンサード リンク
|