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Agnatha (Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
, "no jaws") is a class
Class (biology)

A class is the taxonomic rank in the biological classification of organisms in biology below phylum and above Order .The orders of taxonomy are life, Domain , kingdom , phylum, class , order , family , genus, and species....
 or superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Many recent textbooks regard the group as paraphyletic but recent molecular data, both from rRNA and from mtDNA strongly supports living agnathans as monophyletic. It has existed since the Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
, and continues to live now. There are two extant groups of jawless fish (sometimes called cyclostomes), the lamprey
Lamprey

A lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. While lampreys are well known for those species which bore into the flesh of other fish to hematophagy, these species make up the minority....
s and the hagfish
Hagfish

Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the phylum Vertebrata....
, with about 100 species
Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring....
 in total.






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Agnatha (Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
, "no jaws") is a class
Class (biology)

A class is the taxonomic rank in the biological classification of organisms in biology below phylum and above Order .The orders of taxonomy are life, Domain , kingdom , phylum, class , order , family , genus, and species....
 or superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Many recent textbooks regard the group as paraphyletic but recent molecular data, both from rRNA and from mtDNA strongly supports living agnathans as monophyletic. It has existed since the Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
, and continues to live now. There are two extant groups of jawless fish (sometimes called cyclostomes), the lamprey
Lamprey

A lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. While lampreys are well known for those species which bore into the flesh of other fish to hematophagy, these species make up the minority....
s and the hagfish
Hagfish

Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the phylum Vertebrata....
, with about 100 species
Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring....
 in total. Although they are in the subphylum Vertebrata, hagfish technically do not have vertebrae; they are sometimes classified in Craniata
Craniata

Craniata is a proposed clade of Chordata animals that contains the vertebrates and Myxini as living representatives. Craniata includes all animals with a skull, or cranium, as the name suggests....
. In addition to the absence of jaws, Agnatha are characterised by absence of paired fin
Fin

A fin is a surface used to produce lift and thrust or to steer while traveling in water, air, or other fluid media. The first use of the word was for Fish anatomy#Fins of fish, but has been extended to include other animal limbs and man-made devices....
s; the presence of a notochord
Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cell s derived from the mesoderm and defines the primitive axis of the embryo....
 both in larvae and adults; and seven or more paired gill
Gill

A gill is an anatomical structure found in many aquatic ecosystem organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide....
 pouches. There is a light sensitive pineal eye
Pineal gland

The pineal gland is a small endocrine system gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic functions....
 (homologous to the pineal gland
Pineal gland

The pineal gland is a small endocrine system gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic functions....
 in mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s). All living and most extinct Agnatha do not have an identifiable stomach
Stomach

In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....
 or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic, with a cartilaginous
Cartilage

Cartilage is a type of dense connective tissue. It is composed of specialized cells called chondrocyte that produce a large amount of extracellular matrix composed of collagen fibers, abundant ground substance rich in proteoglycan, and elastin fibers....
 skeleton
Skeleton

In biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa....
, and the heart
Heart

The heart is a muscle organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods....
 contains 2 chambers.

While a few scientists still regard the living agnaths as only superficially similar, and argue that many of these similarities are probably shared basal
Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group form an outgroup to the rest of the clade, such as in the following example:...
 characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than they are to the jawed fishes.

Respiratory system

Agnathans are characterized by seven pairs of gill pouches. The bronchial arches supporting the gill pouches lie close to the body surface.

Metabolism

Agnathans are ectothermic or 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 distinct stomach, rather a long gut more or less homogenous througout its lenght.

Body covering

The only modern Agnathan body covering is skin. There are no scales at all. Many extinct Agnathans had thick body plates (see below).

Appendages

Agnathans have no paired appendages, although they do have a tail
Tail

The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals and birds....
 and a caudal fin.

Skeleton

The internal skeleton of the Agnatha is not bony but rather cartilaginous (made up of dense connective tissue). Also, Agnathans have a notochord
Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cell s derived from the mesoderm and defines the primitive axis of the embryo....
 for their whole life, a characteristic distinctive of the class. This notochord is the first primitive vertebral column
Vertebral column

In human anatomy, the vertebral column is a column of 24 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsum aspect of the torso, separated by spinal discs....
.

Reproduction

Fertilization is external, as is development. There is no parental care.

Fossil agnathans

is a fossil agnathan.]] is another fossil agnathan.]]

Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early Paleozoic
Paleozoic

The Paleozoic or Palaeozoic Era is the earliest of three geology Era of the Phanerozoic Eon . The Paleozoic spanned from roughly , and is subdivided into six period ; from oldest to youngest they are: the Cambrian, Ordovician, Silurian, Devonian period, Carboniferous, and Permian...
. Two types of Early Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
 animal apparently having fins, vertebrate
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
 musculature, and gills are known from the early Cambrian Maotianshan shales
Maotianshan shales

The Maotianshan Shales area a series of lower Cambrian deposits, famous for their Lagerst?tten. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China....
 of China
China

China is a Culture of China, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
: Haikouichthys
Haikouichthys

Haikouichthys is an extinct genus of Craniata believed to have lived c. 530 million years ago, during the Cambrian explosion. Haikouichthys had a defined skull and other characteristics that have led paleontology to label it a true craniate, but it does not possess sufficient features to be included uncontroversially even in the stem gro...
 and Myllokunmingia
Myllokunmingia

Myllokunmingia is a primitive, probably agnathid, jawless fish from the Lower Cambrian Maotianshan shales of China, thought to be a vertebrate, although this is not conclusively proven....
. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella
Haikouella

Haikouella is a probable chordata from the Lower Cambrian Maotianshan shales of Chengjiang County in Yunnan Province, China.It is similar to the form Yunnanozoon, which is almost somewhat more primitive and is possibly a hemichordata....
. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale
Burgess Shale

The Burgess Shale Formation is one of the world's most celebrated fossil localities, and is famous for the exceptional preservation of the fossils found within it, in which the soft parts are preserved....
 of British Columbia
British Columbia

British Columbia is the westernmost of Canada's Provinces and territories of Canada and is famed for its natural beauty, as reflected in its Latin motto, Splendor sine occasu ....
.

Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the Ostracoderm
Ostracoderm

Ostracoderms are any of several groups of extinction, primitive, jawless fishes that were covered in an armor of Bone plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period Stratigraphy of North America and Europe....
s, precursors to the bony fish and hence to the tetrapods (including human
Human

A human being, also human or man, is a member of a species of bipedalism primates in the family Hominidae . Mitochondrial DNA evidence indicates that modern humans originated in east Africa about 200,000 years ago....
s)—are known from the middle Ordovician
Ordovician

The Ordovician is a geologic period, the second of six of the Paleozoic era , and covers the time between 488.3?1.7 to 443.7?1.5 million years ago ....
, and by the Late Silurian
Silurian

The Silurian is a geologic period that extends from the end of the Ordovician period, about 443.7 ? 1.5 annum , to the beginning of the Devonian period, about 416.0 ? 2.8 Mya ....
 the agnathans had reached the high point of their evolution. Agnathans declined in the Devonian
Devonian

The Devonian is a geologic period of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
 and never recovered.

Groups

  • Myxini (hagfish)
  • Hyperoartia
    Hyperoartia

    Hyperoartia is a group of jawless fishes that includes the modern lampreys and their fossil relatives, the jawless fishes of the class Anaspida....
    • Petromyzontidae (lamprey
      Lamprey

      A lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. While lampreys are well known for those species which bore into the flesh of other fish to hematophagy, these species make up the minority....
      s)
  • Pteraspidomorphi
    Pteraspidomorphi

    Pteraspidomorphi is an extinct subclass of early jawless fish. The fossils show extensive shielding of the head. Some species may have lived in fresh water....
  • Thelodonti
    Thelodonti

    There is much debate over whether the clade of Palaeozoic fish known as the Thelodonti represent a Monophyly, or disparate stem groups to the major lines of Agnatha and Gnathostome....
  • Anaspida
    Anaspida

    The Anaspida are stem gnathostomes, and are classically regarded as the ancestors of lampreys. Anaspids were small marine agnathans that lacked scales and paired fins....
  • Cephalaspidomorphi
    Cephalaspidomorphi

    Cephalaspidomorphs are a taxon of agnatha named for the cephalaspids, a group of Osteostraci. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys....
    • Galeaspida
      Galeaspida

      Galeaspida is an extinct taxon of jawless marine and freshwater fish. Their name is derived from a latin word for helmet, galea, and refers to their massive bone shield on the head....
    • Pituriaspida
      Pituriaspida

      The Pituriaspida are a small group of armored jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia ....
    • Osteostraci
      Osteostraci

      The class Osteostraci was a group of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Wenlock epoch to Late Devonian....


See also

  • Gnathostomata
    Gnathostomata

    Gnathostomata is the group of vertebrates with jaws.The group is traditionally a superclass , broken into two top level groupings; cartilaginous fish, and all other members, including the familiar classes of bony fish, birds, mammals, reptiles, and amphibians....