Practice question

Embryological torsion is hallmark of Class Gastropoda occurring during veliger stage, involving 180 degree counterclockwise rotation of visceral mass, mantle and mantle cavity relative to head-foot, bringing mantle cavity and anus anteriorly above head. Facilitates withdrawal into shell for defense, allows head to emerge first when crawling, but imposes sanitation problem where excreta released near head, partially mitigated by asymmetry loss of one ctenidium. Nervous system becomes figure-eight due to visceral loop twisting. Detorsion occurs secondarily in opisthobranchs and nudibranchs. Bivalves, cephalopods and polyplacophorans never undergo torsion, making it gastropod diagnostic apomorphy.