Anamniotes
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 Trouts spawning showing typical anamniote external fertilization
 Anamniotes have a distinct larval stage, here a smooth newt
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The anamniotes are an informal group of vertebrates that lack the amnion during fetal development. These animals are not able to have embryos that develop on land, thus they lay their eggs in water exclusively.[1] During their embryonic development, all anamniote classes pass through a stage which resembles fish, thus indicating their close physiological relationship.
[edit] Anamniote traits
The anamniotes comprises the fishes and the amphibians, the "lower vertebrates", as opposed to the amniotes, the "highter vertebrates". The group is characterized by retaining the primitive vertebrate condition in several traits:[2][3]
- Presence at some period of life of gills
- Absence of an amnion
- Absence or rudimentary condition of the allantois
- Nucleated red blood cells.
[edit] 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.[4] It is a taxonomic classification just below the level of Vertebrata, though Huxley presented the Ichtyopsida 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.[5] 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.
[edit] References
- ^ 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 9780471384618.
- ^ 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
- ^ Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures
- ^ Encyclopaedia Britannica, 9th ed. (1878). original text
