Ichthyopsida is a term coined by Thomas Henry Huxley in 1863. 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.
The ichtyopsids comprising 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:
- Presence at some period of life of gills
- Absence of an amnion
- Absence or rudimentary condition of the allantois
- Nucleated red blood cells.
The group is often referred to as anamniotes due to the lacking the amnion. During their embryonic development, all ichthyopsid classes pass through a stage which resembles the fish, thus indicating their close phylogenetic relationship.
References
Manual of Zoology, by Henry Alleyne Nicholson. William Blackwood And Sons, 1880
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