Monoplacophora

...There are no joints, but the pedal nerves and lateral nerves that extend backwards are connected at 10 places, there are eight pairs of muscles that retract the legs, six pairs of kidneys, two pairs of gonads, two pairs of gill veins, and many repetitive structures in the organs, which shows that it is the closest to annelids among mollusks and is the most primitive. In the past, Paleozoic fossil species of this type were placed in the Gastropoda class of Limpets, but the discovery of modern species showed that it was a completely different group from Gastropods because the left and right nerves do not cross, the mouth is at the front end, and the anus is at the back, and so the class Monoplacophora was established. It is believed that the Gastropoda, Bivalvia, Ceratopoda, and Cephalopoda classes evolved from this. ...

*Some of the terminology that mentions "Monoplacophora" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…関節はないが後方へのびる足神経と側神経は10ヵ所で連絡し，足を縮める筋肉は8対あり，腎臓は6対，生殖腺は2対，えら静脈は2対など器官に多くの繰返し構造があって，軟体動物の中でもっとも環形動物に近いことを示し，もっとも原始的である。かつてはこの類の古生代の化石種は腹足綱のカサガイ類に入れられていたが，現生種の発見によって左右の神経が交叉していないこと，口が前端，肛門が後方にあるなどによって，腹足類とまったく異なる群であることがわかり，単板綱Monoplacophoraが確立した。腹足綱，二枚貝綱，ツノガイ綱，頭足綱などはこれから分化したとされる。…

※「Monoplacophora」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報