Trouts spawning showing typical anamniote external fertilization
|Cladistically included but traditionally excluded groups|
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. 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. During their development, all anamniote classes pass through a stage which resembles fish, thus indicating their close physiological relationship.
- 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.
History of discovery
The features unifying the anamniotes was first noted by Thomas Henry Huxley in 1863, who coined the phrase Ichtioid or Ichthyopsida ("fish-face") for the group. It is a taxonomic classification just below the level of Vertebrata, though Huxley presented the Ichthyopsida as an informal unit and never ventured to forward a Linnaean rank for the group. The term ichthyopsida means fish-face or fish-like as opposed to the sauropsida or lizard-face animals (reptiles and birds) and the mammals. The group representing an evolutionary grade rather than a clade, the term anamniote is now used as an informal way of denoting the physical property of the group, rather than as a systematic unit.
- Colbert, E. H.; Morales, M. (2001). Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time (4th ed.). New York: John Wiley & Sons. ISBN 978-0-471-38461-8.
- Carroll, R. L. (1991). "The origin of reptiles". In Schultze, H.-P.; Trueb, L. Origins of the higher groups of tetrapods — controversy and consensus. Ithaca: Cornell University Press. pp. 331–353. ISBN 0-8014-2497-6.
- Romer, A. S.; Parsons, T. S. (1985) . The Vertebrate Body (6th ed.). Philadelphia: Saunders. ISBN 0-03-058443-4.
- Nicholson, H.A. (1880): Manual of Zoology, Blackwood And Sons. Original text
- Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures
- Encyclopædia Britannica, 9th ed. (1878). original text