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Torsion and detorsion in gastropods
Torsion is a defining, and rather dramatic, developmental event unique to gastropods, occurring during their larval stage. It involves a counter-clockwise (typically) twisting of the visceral mass, mantle, and mantle cavity by 90 to 180 degrees relative to the head and foot. This rotation brings the anus and mantle cavity, containing the gills, forward, often positioning them over the head. While seemingly awkward, it's thought to offer adaptive advantages, such as allowing the head to retract into the shell first for protection and providing clean water to the gills by facing them forward.
Detorsion, conversely, is the partial or complete reversal of this torsion, occurring in some gastropod lineages. Organisms exhibiting detorsion will see their anus and mantle cavity shift back towards a more posterior or lateral position. This secondary untwisting is an evolutionary modification, suggesting that the initial advantages of torsion might be mitigated by new selective pressures or that detorsion itself provides new benefits, such as reduced fouling of the gills by waste products.
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