Detorsion is reversal of torsion which takes place when during evolution shell is lost or a type of shell evolves that has openings on the opposite sides. In such situations twisting of visceral mass is not necessary. Hence, detorsion takes place during the larval stage and the animal again becomes bilaterally symmetrical. Nervous system becomes symmetrical and not twisted in the shape of 8. Pallial complex travels backwards. Ctenidium travels backwards or to the lateral side. Auricle moves behind the ventricle. Visceral loop and intestine become straightened.
Detorsion takes place in Pulmonata, such as Acteon and Bulla in which anus and ctenidium are directed laterally. In Aplysia (Opisthobranchia), detorsion takes place owing to the loss of shell. The gills are directed laterally but lie posterior to the heart and the body becomes symmetrical.
Nudibranchs (Eolis and Doris) also undergo detorsion due to the loss of shell. Doris has symmetrical rhinophores and anal gills on the posterior side. The sea slugs, Eolis and Iolidia are symmetrical animals because they have undergone detorsion due to the absence of shell. They move with the help of a ventral foot and breathe with respiratory cerata that are present all over the body.
The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling (Figure 5). Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia as well as a pair of nephridia. 1e1e36bf2d