1.NounA plesiomorphic condition present in some gastropods, involving torsion of the internal organs from left to right in order to accommodate the relocation of the mantle cavity close to the head, leading to streptoneury, followed by the reversal of this process, straightening out the internal organs and uncrossing the commissures in the process.
"The Opisthobranchia are one of the more prominent gastropod lineages that have undergone euthyneury."