Difference between revisions of "Agnatha" - New World Encyclopedia

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Agnatha generally is considered to by paraphyletic. Similarities between hagfish and lampreys appear to involve superficial or primitive characteristics that cannot support a hypothesis for a group composed of only hagfish and lampreys as a monophyletic group (Nelson 1994). On the other hand, there are a number of morphological and physiological similarities shared between lampreys and gnathostomes, but not hagfishes, that appear to be due to common ancestry, such as vertebral elements, highly differentiated kidney tubules, more than one semicircular canal, large exocrine [[pancreas]], and so forth (Nelson 1994). Yalden (1985), however, does present an argument based on feeding mechanisms that hagfishes and lampreys do constitute a monophyletic grouping.
 
Agnatha generally is considered to by paraphyletic. Similarities between hagfish and lampreys appear to involve superficial or primitive characteristics that cannot support a hypothesis for a group composed of only hagfish and lampreys as a monophyletic group (Nelson 1994). On the other hand, there are a number of morphological and physiological similarities shared between lampreys and gnathostomes, but not hagfishes, that appear to be due to common ancestry, such as vertebral elements, highly differentiated kidney tubules, more than one semicircular canal, large exocrine [[pancreas]], and so forth (Nelson 1994). Yalden (1985), however, does present an argument based on feeding mechanisms that hagfishes and lampreys do constitute a monophyletic grouping.
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There area bout 12 genera and84 species recognized in the Agnatha (Nelson 1994).
  
 
==Description==
 
==Description==
  
Members of Agnatha are characterized by the absence of jaws derived from gill arches (although hagfish and some fossil forms have a biting apparatus that is not considered to have been derived from gill arches) (Nelson 1994). Other common characteristics include the absence of paired [[fin]]s, absence of pelvic fins, the presence of a [[notochord]] both in larvae and adults, and seven or more paired [[gill]] pouches. There is a lack of a vertebral centra, presence of one or two vertical semicircular canals, the gills are directed internally and covered with endoderm, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (Nelson 1994). There is a light sensitive [[Pineal_gland#In_lower_vertebrates|pineal eye]] (homologous to the [[pineal gland]] in [[mammal|mammals]]).  
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Members of Agnatha are characterized by the absence of jaws derived from gill arches, although hagfish and some [[fossil]] forms have a biting apparatus that is not considered to have been derived from gill arches (Nelson 1994). Other common characteristics of Agnatha include the absence of paired [[fin]]s, absence of pelvic fins, the presence of a [[notochord]] both in larvae and adults, and seven or more paired [[gill]] pouches. There is a lack of a vertebral centra, presence of one or two vertical semicircular canals, the gills are directed internally and covered with endoderm, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (Nelson 1994). The bronchial arches supporting the gill pouches lie close to the body surface. There is a light sensitive [[Pineal_gland#In_lower_vertebrates|pineal eye]] (homologous to the [[pineal gland]] in [[mammal|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 extant members of the Agnatha class.  The Agnatha are [[ectothermic]], with a [[Cartilage|cartilaginous]] [[skeleton]], and the [[heart]] contains 2 chambers.
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All living and most extinct agnathans do not have an identifiable [[stomach]] or any paired [[appendage]]s, although the hagfish and lampreys do have a [[tail]] and a [[caudal fin]]. Both hagfish and lamprey have slimy skin without scales or plates. Some extinct agnathans reveal thick body plates. The internal skeleton of the Agnatha is not bony but rather [[cartilaginous]] (made up of dense connective tissue).
  
Both hagfish and lamprey have slimy skin without scales or plates. They also have a [[notochord]] that remains throughout life; circular, jawless mouths; and unpaired fins.  
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As characteristic of the class, hagfish and lampreys have a notochord that remains throughout life. This notochord is the first primitive [[vertebral column]]. In the extant agnathans, fertilization and development are both external, and there is no parental care. The lampreys and hagfish have circular, jawless mouths and unpaired fins. They are [[ectothermic]], with a [[Cartilage|cartilaginous]] [[skeleton]], and the [[heart]] contains 2 chambers. Being [[ectothermic]] or cold blooded, they do not have to warm themselves through eating. Therefore, their metabolism is slow as well and they do not have to eat as much.  
  
 
Although lampreys and hagfish are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates. Thus, modern classifications tend to place hagfish into a separate group (such as the Myxini or Hyperotreti), with the [[lamprey]]s (Hyperoartii) being more closely related to the jawed fishes.
 
Although lampreys and hagfish are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates. Thus, modern classifications tend to place hagfish into a separate group (such as the Myxini or Hyperotreti), with the [[lamprey]]s (Hyperoartii) being more closely related to the jawed fishes.
  
 
===Hagfish===
 
===Hagfish===
Hagfish are found in the oceans and lampreys are found in both freshwater and ocean environments. Most lampreys are parasitic.
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Hagfish, placed in order Myxiniformes (Hyperotreti) or subphylum Myxini, are found in the oceans. They have a scaleless, eel-like body without paired fins. They are characterized by one semicircular canal, a single olfactory capsule with folds in sensory epithelium, no bone, absence of eye musculature, and 1 to 16 pairs of external gill openings (Nelson 1994). Sometimes known as "slime eels," they are a staple food in [[Korea]].  
 
 
  
 
===Lampreys===
 
===Lampreys===
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Lampreys are found in both freshwater and ocean environments, being anadromous. Most are parasitic. The extant lampreys, placed in the family Petromyzontidae of the order Petromyzontiformes (Hyperoartii), are characterized by one or two dorsal fins, well developed eyes in adults, teeth on the oral disc and tongue (although not fossil forms), separate sexes, and a larval stage that undergoes a radical metamorphosis in freshwater (Nelson 1994).
  
 
For this reason, hagfish, which are also commonly known as "slime eels," are sometimes not considered to be fish. The other living member of Agnatha, the [[lamprey]], has primitive vertebrae made of cartilage. Hagfish are a staple food in [[Korea]]. They are classified in the order Myxini and the family Myxinidae.
 
 
==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==
 
==Fossil agnathans==

Revision as of 23:47, 31 May 2008

Idealized bauplan of the Agnatha.

Agnatha is a chordate taxonomic group comprising the jawless fish and generally is placed as a superclass within in the subphylum Vertebrata. Agnathans have existed since the Cambrian and continue to be present now, with two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish.

Characterized by the absence of jaws derived from gill arches, agnathans are differentiated from the gnathostomes (superclass Gnathostomata), or "jawed vertebrates" (fish with hinged jaws, amphibians, reptiles, birds, and mammals). However, Agnatha generally is considered to be a paraphyletic grouping (Purnell et al. 2001; Nelson 1994).

Overview and classification

Vertebrates (subphylum Vertebrata) are generally classified into two groups, the Agnatha (jawless vertebrates) and the Gnathostomata (jawed vertebrates). The later group includes fish with hinged jaws and the tetrapods (amphibians, reptiles, birds, and mammals). Agnatha (Greek, "no jaws") includes the modern day lampreys (Petromyzontiformes) and hagfish (Myxiniformes) as well as several extinct orders.

In typical biological classifications, Agnatha and Gnathostomata are each considered a superclass of Vertebrata. However, there are different taxonomies, including ones in which Agnatha is considered a class, or Gnathostomata is not recognized as a taxon (ITIS 2001), or Agnatha is not recognized (Janvier 1981).

Hagfish, while generally classified in Agnatha and in the subphylum Vertebrata, actually lack vertebrae. For this reason, they sometimes are separated from the vertebrates. Janvier (1981) and others, for example, put hagfish as another subphylum, Myxini, along with the subphylum Vertebrata, under Craniata, recognizing the common possession of a cranium (Janvier 1981). Others, however, use the terms Vertebrata and Craniata as synonyms, rather than different levels of classification, and retain the use of Agnatha (Nelson 1994).

The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage.

Agnatha generally is considered to by paraphyletic. Similarities between hagfish and lampreys appear to involve superficial or primitive characteristics that cannot support a hypothesis for a group composed of only hagfish and lampreys as a monophyletic group (Nelson 1994). On the other hand, there are a number of morphological and physiological similarities shared between lampreys and gnathostomes, but not hagfishes, that appear to be due to common ancestry, such as vertebral elements, highly differentiated kidney tubules, more than one semicircular canal, large exocrine pancreas, and so forth (Nelson 1994). Yalden (1985), however, does present an argument based on feeding mechanisms that hagfishes and lampreys do constitute a monophyletic grouping.

There area bout 12 genera and84 species recognized in the Agnatha (Nelson 1994).

Description

Members of Agnatha are characterized by the absence of jaws derived from gill arches, although hagfish and some fossil forms have a biting apparatus that is not considered to have been derived from gill arches (Nelson 1994). Other common characteristics of Agnatha include the absence of paired fins, absence of pelvic fins, the presence of a notochord both in larvae and adults, and seven or more paired gill pouches. There is a lack of a vertebral centra, presence of one or two vertical semicircular canals, the gills are directed internally and covered with endoderm, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (Nelson 1994). The bronchial arches supporting the gill pouches lie close to the body surface. There is a light sensitive pineal eye (homologous to the pineal gland in mammals).

All living and most extinct agnathans do not have an identifiable stomach or any paired appendages, although the hagfish and lampreys do have a tail and a caudal fin. Both hagfish and lamprey have slimy skin without scales or plates. Some extinct agnathans reveal thick body plates. The internal skeleton of the Agnatha is not bony but rather cartilaginous (made up of dense connective tissue).

As characteristic of the class, hagfish and lampreys have a notochord that remains throughout life. This notochord is the first primitive vertebral column. In the extant agnathans, fertilization and development are both external, and there is no parental care. The lampreys and hagfish have circular, jawless mouths and unpaired fins. They are ectothermic, with a cartilaginous skeleton, and the heart contains 2 chambers. Being ectothermic or cold blooded, they do not have to warm themselves through eating. Therefore, their metabolism is slow as well and they do not have to eat as much.

Although lampreys and hagfish are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates. Thus, modern classifications tend to place hagfish into a separate group (such as the Myxini or Hyperotreti), with the lampreys (Hyperoartii) being more closely related to the jawed fishes.

Hagfish

Hagfish, placed in order Myxiniformes (Hyperotreti) or subphylum Myxini, are found in the oceans. They have a scaleless, eel-like body without paired fins. They are characterized by one semicircular canal, a single olfactory capsule with folds in sensory epithelium, no bone, absence of eye musculature, and 1 to 16 pairs of external gill openings (Nelson 1994). Sometimes known as "slime eels," they are a staple food in Korea.

Lampreys

Lampreys are found in both freshwater and ocean environments, being anadromous. Most are parasitic. The extant lampreys, placed in the family Petromyzontidae of the order Petromyzontiformes (Hyperoartii), are characterized by one or two dorsal fins, well developed eyes in adults, teeth on the oral disc and tongue (although not fossil forms), separate sexes, and a larval stage that undergoes a radical metamorphosis in freshwater (Nelson 1994).


Fossil agnathans

Haikouichthys is a fossil agnathan.
Cephalaspis is another fossil agnathan.

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
  • Pteraspidomorphi
  • Thelodonti
  • Anaspida
  • Cephalaspidomorphi
    • Galeaspida
    • Pituriaspida
    • Osteostraci

References
ISBN links support NWE through referral fees


  • Integrated Taxonomic Information System (ITIS). 2003. Agnatha ITIS Taxonomic Serial No.: 159693. Retrieved May 31, 2008.
  • Integrated Taxonomic Information System (ITIS). 2001. Vertebrata ITIS Taxonomic Serial No.: 331030. Retrieved May 31, 2008.
  • Janvier, P. 1981. The phylogeny of the Craniata, with particular reference to the significance of fossil "agnathans." J. Vertebr. Paleont. 1(2):121-159.
  • Nelson, J. S. 1994. Fishes of the World, 3rd ed. New York: John Wiley & Sons. ISBN 0471547131.

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}

  • Yalden, D. W. 1985. Feeding mechanisms as evidence for cyclostome monophyly. Zool. J. Linn Soc. 84:291-300.

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