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Craniate
Craniata (sometimes Craniota) is a proposed clade of chordate animals that contains the Myxini (hagfish), Petromyzontida (including lampreys), and Gnathostomata (jawed vertebrates)〔Campbell & Reece 2005 p. 676〕〔Cracraft & Donoghue 2004 p. 390〕 as living representatives. As the name suggests, Craniata are animals with a (hard bone or cartilage) skull in Chordata.
==Characteristics==
In the simplest sense, craniates are chordates with heads, thus excluding members of the chordate subphyla Tunicata (tunicates) and Cephalochordata (lancelets), but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin. Craniata also includes all lampreys and armored jawless fishes, armoured fish, sharks, skates, and rays, and Teleostomians: spiny sharks, bony fish, lissamphibians, temnospondyls and protoreptiles, Sauropsids and mammals. The craniate head consists of a brain, sense organs, including eyes, and a skull.〔Campbell & Reece 2005 pp. 675-7〕〔Parker & Haswell 1921〕
In addition to distinct crania (sing. ''cranium''), craniates possess many derived characteristics, which have allowed for more complexity to follow. Molecular-genetic analysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of many gene families that are involved in cell signaling, transcription, and morphogenesis (see homeobox).〔
In general, craniates are much more active than tunicates and lancelets and, as a result, have greater metabolic demands, as well as several anatomical adaptations. Aquatic craniates have gill slits, which are connected to muscles and nerves that pump water through the slits (as opposed to lancelets, whose pharyngeal slits are used only for suspension feeding), engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving food through the canal, allowing higher craniates like mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems that include a heart with two or more chambers, red blood cells, and O2 transporting hemoglobin, as well as kidneys.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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