Sphaeromidae

...A small crustacean with an elliptical body, flattened dorsally and ventrally, and about 5 to 15 mm long. The Japanese name given to several species of Sphaeromidae that live on the coast, and the general name for all Sphaeromidae members. All Sphaeromidae members have a flat apical limb and abdominal telson (the posterior abdominal segment and telson fused together) that form a tail fan, and they can roll their bodies ventrally like terrestrial pillbugs. They stick their flattened bodies tightly against other objects, but they are also very skilled swimmers, swimming on their backs. ...

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

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…体は楕円形，背腹に扁平，長さ5～15mm前後の小型の甲殻類。等脚目コツブムシ科Sphaeromidaeに属するもののうち磯にすむ数種に与えられた和名およびこの科に属するものの総称，コツブムシ科のものは，すべて扁平な尾肢と腹尾節(後方の腹節と尾節が融合したもの)とで尾扇を形成し，陸生のダンゴムシの類のように体を腹側に丸めることができる。扁平な体を他の物に密着させてはうが，泳ぐのも非常に達者で，背を下にして泳ぐ。…

※「Sphaeromidae」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