Agnatha

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Agnatha

[′ag·nə′thə]
(vertebrate zoology)
The most primitive class of vertebrates, characterized by the lack of true jaws.

Agnatha

 

a superclass of lower vertebrate animals. Agnatha are distinguished from all the remaining vertebrates, the Gnathostomata, by the absence of real jaws, and, in the ones living today, by the absence of paired extremities, as well as by the presence of an unpaired nostril. Agnatha are also called Marsipobranchia, because their gills look like pouches. The branchiate skeleton is located outside the pouches and has the appearance of a complete lattice (and not broken gill arches, as in fish) or is fused with the external shell. Agnatha is the most ancient group of vertebrates, widely distributed in the Silurian and Devonian periods. Fossil Agnatha (Ostracodermi) had a well-developed external and partially ossified internal skeleton. Their remains are the leading fossils for the Silurian and Devonian periods.

Of contemporary fauna, only representatives of the class Cyclostomata—the lampreys and hagfish—remain of the Agnatha.

REFERENCES

Berg, L. S. Sistema ryb. Moscow-Leningrad, 1940.
Osnovy paleontologii: Bescheliustnye, ryby. Moscow, 1964.
References in periodicals archive ?
The two main taxonomic groups of agnathan or cyclostome fish are Hyperoartia, lampreys and Hyperotreti, and hagfish (for a review of agnathan ion and water homeostasis see Karnaky, 1998; Evans and Claiborne, 2009).
Earlier invertebrate species do not possess either gene, and thus these two aqp genes, which are derived from aqp4, likely originated in an agnathan ancestor.
The Boothiaspidinae, a new agnathan subfamily (Heterostraci, Cyathaspididae) from the Late Silurian and Early Devonian of the western United States and the Canadian Arctic.
In Scotland, the associated faunal list includes the protochordate Ainiktozoon loganense Scourfield and the agnathan Jamoytius kerwoodi White in addition to L.
Silurian-Devonian vertebrate dominated communities, with particular references to agnathans.
Gross (1947) studied fish microremains from the erratic boulders of the North German Lowland, among other agnathans and fishes also Strosipherus indentatus.
Isolation of Dlx and Emx gene cognates in an agnathan species, Lampetra japonica, and their expression patterns during embryonic and larval development: conserved and diversified regulatory patterns of homeobox genes in vertebrate head evolution.