Agnatha (Greek, "no jaws") is a
paraphyletic[cite book|last=Purnell|first=M. A.|authorlink=|editor=Derek E. G. Briggs and Peter R. Crowther|year=2001|title=Palaeobiology II|publisher= Blackwell Publishing|location= Oxford|isbn=0-632-05149-3|page=p401; Note, however, there also is recent molecular evidence for monophyly. ] superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. It has existed since the Cambrian, and continues to live now. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 100 speciesin total. Although they are in the subphylum Vertebrata, hagfish technically do not have vertebrae; they are sometimes classified in Craniata. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochordboth in larvae and adults; and seven or more paired gillpouches. There is a light sensitive pineal eye (homologous to the pineal glandin mammals). All living and most extinct Agnatha do not have an identifiable stomachor any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic, with a cartilaginous skeleton, and the heartcontains 2 chambers.
Although they are superficially similar, many of these similarities are probably shared basal characteristics of ancient vertebrates, and modern classifications tend to place hagfish into a separate group (the Myxini or Hyperotreti), with the
lampreys(Hyperoartii) being more closely related to the jawed fishes.
Agnathans are characterized by seven pairs of gill pouches. The bronchial arches supporting the gill pouches lie close to the body surface.
ectothermicor cold blooded, meaning they do not have to warm themselves through eating. Therefore, Agnathan metabolism is slow as well as the fact that Agnathans do not have to eat as much. They have no stomach.
The only modern Agnathan body covering is skin. There are no scales. Many extinct Agnathans had thick body plates (see below).
Agnathans have no paired appendages, although they do have a
tailand a caudal fin.
The internal skeleton of the Agnatha is not bony but rather
cartilaginous(made up of dense connective tissue). Also, Agnathans have a notochordtheir whole life, a characteristic distinctive of the class. This notochord is the first primitive vertebral column.
Fertilization is external, as is development. There is no parental care.
Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early
Paleozoic. Two types of Early Cambriananimal apparently having fins, vertebratemusculature, and gills are known from the early Cambrian Maotianshan shalesof China: " Haikouichthys" and " Myllokunmingia". They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is " Haikouella". A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shaleof British Columbia.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the
Ostracoderms, precursors to the bony fishand hence to the tetrapods(including humans)—are known from the middle Ordovician, and by the Late Silurianthe agnathans had reached the high point of their evolution. Agnathans declined in the Devonianand never recovered.
** Petromyzontidae (
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Agnatha — n. a class of eel shaped chordates with a cartilaginous skeleton lacking jaws, scales, and pelvic fins. Among these are the lampreys and hagfishes. There are some extinct forms. Syn: class Agnatha. [WordNet 1.5] … The Collaborative International Dictionary of English
Agnătha — Agnătha, bei Dumer. die Gattungen Phryganea u. Ephemera. L … Pierer's Universal-Lexikon
Agnatha — Agnatha, die Kieferlosen ( Fischartige ). * * * Agna|tha <Pl.> [zu griech. a = nicht, un u. gnáthos = [Kinn]backe (Zool.): im Wasser lebende fischähnliche Wirbeltiere, die keinen Kiefer haben … Universal-Lexikon
Agnatha — Agnatha, Kieferlose, den ⇒ Gnathostomata gegenübergestellte ca. 75 marine und limnische Arten umfassende Überklasse sehr ursprünglicher Wirbeltiere. Kiefer fehlen; Saugmund mit Hornzähnchen; äußere unpaare Öffnung des Nasen Hypophysengangs;… … Deutsch wörterbuch der biologie
Agnatha — Agnathes … Wikipédia en Français
Agnatha — Peces sin mandibulas Rango temporal: Cámbrico Presente … Wikipedia Español
Agnatha — Conodonta (extintos) Hyperoartia Myxinoidea Petromyzontidae Anaspida (extintos) Thelodonti (extintos) Pteraspidomorphi (extintos) Thelodontida (extintos) Cephalaspidomorphi (extintos) Galeaspida (extintos) Pituriaspida (extintos) Osteostraci… … Enciclopedia Universal
Agnatha — Für die systematische Einteilung der Lebewesen existieren nebeneinander mehrere Vorschläge. Das hier behandelte Taxon entspricht nicht der gegenwärtig in der deutschsprachigen Wikipedia verwendeten Systematik. Flussneunauge Die Kieferlosen… … Deutsch Wikipedia
Agnatha — noun superclass of eel shaped chordates lacking jaws and pelvic fins: lampreys; hagfishes; some extinct forms • Syn: ↑superclass Agnatha • Hypernyms: ↑class • Member Holonyms: ↑Vertebrata, ↑subphylum Vertebrata, ↑ … Useful english dictionary
Agnatha — … Википедия