Agnatha

(redirected from agnathans)
Also found in: Dictionary, Thesaurus, Medical.

Agnatha

[′ag·nə′thə]
(vertebrate zoology)
The most primitive class of vertebrates, characterized by the lack of true jaws.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Finally, more studies of aquaporins in the kidneys of agnathans and elasmobranchs would allow comparative hypotheses to be tested regarding the role and evolution of aquaporins in fish kidneys.
Origins of the descending spinal projections in petromyzontid and myxinoid agnathans. J.
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.
According to him, no real sensory line canal or groove was visible on the outer surface, but the row of foramina might have affected the conditions of a pit-line and could be the homologue of the ventral longitudinal sensory line of some agnathans (Janvier 1978, p.
In Scotland, the associated faunal list includes the protochordate Ainiktozoon loganense Scourfield and the agnathan Jamoytius kerwoodi White in addition to L.
Because the phylogeny, as well as the fertilization, of living agnathans (hagfish and lampreys) has attracted much attention from biologists (Brodal and Fange, 1963; Hardisty, 1979), we compared these sperm to those of other chordates.
Patten (e.g., 1931), produced additional specimens, exhausting the quarry; in recent decades, no articulated agnathans have been found apart from a few shields of Tremataspis mammillata Patten and Dartmuthia gemmifera Patten.
The fossil fishes that have so far been used for distinguishing zones are agnathans, placoderms and acanthodians (e.g.
Such a very fine pattern has been observed on the scales of both agnathans and gnathostome fishes (Marss 2006) in the Palaeozoic, and some details of the external surface of scales and tubercles have been illustrated in publications on histology (e.g.