Cyclostomium - Cyclostomium

A classification name used for extant species in the Agnatha superclass. In animal taxonomy, it was a general term for the most primitive fishes belonging to the Lampreys and Hagfishes of the Phylum Chordata, but as a classification system was established that included fossil species, the Lampreys were placed in the Class Scaphopoda and the Hagfishes in the Class Myxiniformes. The term Cyclostomata, which includes both classes, is not an official classification name, but is sometimes used conventionally as a convenient term. However, the classification system for this class varies depending on the researcher, and is extremely fluid. The body of this class is eel-shaped and the skin is smooth. The mouth is round and sucker-like, and does not have distinct jaws. The gills are pouch-shaped and there are 5 to 15 pairs, and the external gill slits are 1 to 15 pairs. There are dorsal and caudal fins, but no pectoral or pelvic fins. Although they are classified as fish, they are very different from modern fishes in terms of characteristics such as the lack of bones in their jaws and unpaired external nostrils and gonads. Each member of this group can be called a "living fossil" in that they still retain the characteristics of fish that flourished in the Paleozoic era. They are found in freshwater and marine environments, attaching themselves to fish and large aquatic animals, eating them by scraping off their flesh, and sucking out their body fluids. This can cause large-scale deaths of useful fish such as salmon and trout.

[Akira Ochiai and Kunio Amano, June 17, 2015]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

無顎(むがく)上綱Agnathaのなかの現生種に対して用いられていた分類名。動物分類学上、脊索(せきさく)動物門のヤツメウナギ類とヌタウナギ類に属するもっとも原始的な魚類の総称であったが、化石種を含めた分類体系を構築するなかで、ヤツメウナギ類は頭甲綱に、ヌタウナギ類はヌタウナギ綱になった。両類をまとめた円口類は、正式な分類名ではないが、便利な用語として慣習的に使われることがある。しかしこの類の分類体系は研究者によって異なり、きわめて流動的である。この類の体はウナギ型で皮膚は滑らかである。口は円形で吸盤状で、明瞭(めいりょう)な両あごをもたない。えらは袋状で5～15対、外鰓孔(がいさいこう)は1～15対。背びれと尾びれはあるが、胸びれや腹びれはない。魚類に含まれているが、あごに骨がなく、外鼻孔や生殖腺(せん)が無対であるなどの特徴から、現生の魚類とは大きく異なる。この類はそれぞれで古生代に栄えた魚類の特徴を現在も保持している点で「生きた化石」といえる。淡水や海洋に分布し、魚類や大形の水生動物に吸着し、肉をそぎ取るように食べるほか、体液も吸う。このため、サケ・マス類など有用魚類を大量に死滅させることがある。

［落合　明・尼岡邦夫　2015年6月17日］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例