Agnatha

Agnatha (Greek, "no jaws") is a paraphyletic 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 species in 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 notochord both in larvae and adults; and seven or more paired gill pouches. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach 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 skeleton, and the heart contains 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.

Respiratory system
Agnathans are characterized by seven or more 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 stomach.

Body covering
The only modern Agnathan body covering is skin. There are no scales. Extinct Agnathans had thick body plates (see below).

Appendages
Agnathans have no paired appendages, although they do have a tail 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 their whole life, a characteristic distinctive of the class. This notochord is the first primitive vertebral column.

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

Fossil agnathans
Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal apparently having fins, vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of 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 Shale of 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 fish and hence to the tetrapods (including humans)—are known from the middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution. Agnathans declined in the Devonian and never recovered.

Groups

 * Myxini (hagfish)
 * Hyperoartia
 * Petromyzontidae (lampreys)
 * Pteraspidomorphi
 * Thelodonti
 * Anaspida
 * Cephalaspidomorphi
 * Galeaspida
 * Pituriaspida
 * Osteostraci