Agnatha(redirected from agnathan)
Also found in: Dictionary, Thesaurus, Medical.
Related to agnathan: Osteichthyes
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.
REFERENCESBerg, L. S. Sistema ryb. Moscow-Leningrad, 1940.
Osnovy paleontologii: Bescheliustnye, ryby. Moscow, 1964.