Opisthobranchia

Japanese: Opisthobranchia

...During development, the body twists 180 degrees, nerve trunks cross, the mantle cavity opens forward, and the gills are located in front of the heart. (b) Opisthobranchia: Most of the shell is degenerate or absent. The body returns to its former twist, and the nerves do not cross. ...

From [Gastropods]

...They are mainly marine, but also live in freshwater and on land. (2) Opisthobranchia The shell is usually absent or degenerate, except for the oysters, the oysters, and the oysters. Many of them lack gills, and as the name Opisthobranchia suggests, they have secondary gills around the anus behind the heart.

*Some of the terminology explanations that mention "Opisthobranchia" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…発生の途中で体が180度ねじれて神経幹が交叉し，外套腔は前方に開き，中のえらは心臓より前に位置する。(b)後鰓亜綱Opisthobranchia　多くは殻は退化的あるいは欠いている。体は前者のねじれがもどり神経は交叉しない。…