# 圆口纲[编辑]

| 圆口纲 化石时期：法门期 - 現代
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| --- |
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| 科学分类  |
| 界： | 动物界 Animalia |
| 门： | 脊索动物门 Chordata |
| 演化支： | 嗅球類 Olfactores |
| 亚门： | 脊椎动物亚门 Vertebrata |
| 纲： | **圆口纲 Cyclostomata** Duméril, 1806 |
| 亚纲
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| * 七鳃鳗亚纲 Hyperoartia
* 盲鳗亚纲 Myxini
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**圆口纲**（学名：Cyclostomata）是在动物分类学上提出的一種分類，與已經滅絕的甲冑魚類（Ostracoderms）同位於無頷總綱（Agnatha）之下。現存代表動物為七鰓鰻（又名：八目鰻）。

## 特徵

圆口纲無真正上下頜。口為吸盤型，故稱之無頜類。無成對的附肢。（脊椎動物門中唯一沒有附肢的動物。）終生保留脊索，沒有真正的脊椎骨，只有一些軟骨小弧片直立於脊索上方及神經管的兩側，是脊椎的雛形。（脊椎動物笫一階段進化的代表特徵。）具有獨特的呼吸器官──鰓囊，囊壁為由內胚層來源的褶皺狀鰓絲，遍佈大量的微血管，可供氣體交換。

## 分類

现存种类包括：

* 七鳃鳗亚纲 Hyperoartia
* 盲鳗亚纲 Myxini

這一理論存在爭議。根據各種遺傳學、古生物學研究，盲鰻被認爲與脊椎動物距離較遠，而被劃到有頭動物之下，但另一些研究卻顯示盲鰻與七鰓鰻的關聯較與有頜類的關聯更密切，有可能構成一個單系群，而七鰓鰻與頭甲魚的相似特徵則被認爲可能是趨同演化的結果。

## 種系發生學

下面的演化樹是由宮下哲人等人於2019年系統分析的結果：

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†海口蟲屬 *Haikouella*  |
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†海口魚屬 *Haikouichthys*  |
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†昆明魚屬 *Myllokunmingia*  |
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†後斯普里格魚屬 *Metaspriggina*  |
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| 脊椎動物
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有頜下門  |
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| †缺甲魚綱
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†*Cornovichthys*  |
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†*Achanarella*  |
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†*Ciderius* |
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†Birkeniida

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†*Lasanius*  |
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†*Euphanerops* |
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†*Jamoytius* |
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†*Pipiscius*  |
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†牙形石綱  |
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| 圓口綱
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†似盲鰻屬 *Myxinikela*  |
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| 盲鰻總科
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†特提斯盲鰻屬 *Tethymyxine tapirostrum*  |
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塔斯曼海紅盲鰻 *Rubicundus eos* |
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美洲紅盲鰻 *Rubicundus lopheliae* |
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大西洋盲鰻 *Myxine glutinosa*  |
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新盲鰻 *Neomyxine biniplicata* |
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太平洋黏盲鰻 *Eptatretus stoutii*  |
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蒲氏黏盲鰻 *Eptatretus burger]*  |
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副盲鰻 "*Paramyxine*" spp.

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†*Gilpichthys*  |
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†哈迪斯蒂鳗属 *Hardistiella* |
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†*Mayomyzon*  |
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†*Myxineidus*  |
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†*Priscomyzon*  |
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†中生鳗属 *Mesomyzon*  |
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| 七鰓鰻目
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澳洲囊口七鰓鰻 *Geotria australis*  |
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短頭袋七鰓鰻 *Mordacia mordax*  |
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海七鰓鰻 *Petromyzon marinus*  |
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歐洲七鰓鰻 *Lampetra fluviatilis*  |
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東亞叉牙七鰓鰻 *Lethenteron camtschaticum'*  |
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| (冠群)
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* 有頭動物 Craniata
* 有頜類 Gnathostomata
* 頭甲魚綱 Cephalaspidomorphi
* 鰭甲魚綱 Pteraspidomorphi

## 參考文獻