Practice question

Gastropods are fundamentally bilaterally symmetrical larvae but become secondarily asymmetrical through embryonic torsion occurring during veliger stage, involving 180-degree anticlockwise rotation of visceropallium, mantle and shell relative to head-foot axis. Detorsion secondarily reduces it in opisthobranchs and pulmonates. Torsion relocates mantle cavity from posterior to anterior dorsal position above head, induces chiastoneury crossing pleurovisceral connectives into figure eight, causes loss of right ctenidium and kidney. Differential growth and asymmetric retractor muscle contraction drive this morphogenetic event generating adult asymmetry characteristic of snails and slugs.