Archerd Shell Collection > Shell Classes > Torsion Illustration
Torsion is most readily understood looking at the body plan of an evolutionarily "primitive" mollusc, Neopilina galatheae (a Monoplacophore). In the schematic diagram above, rotation of the straight line body, and a subsequent twisting known as torsion, is illustrated as a progressive transition. Transition is shown from the straight line body plan of the primitive mollusc to a fully torted body plan found in Caenogastropod snails; for example, the cone shell mollusc.
In stage 1, above, paired visceral organs lie on either side of a straight line gut; these include, for example, mucus (salivary) glands, eyes, nervous ganglia, heart, gills, kidney, and gonads. Torsion begins in stages 2 and 3, with a partial rotation of the gut. As the shell begins to coil, a mantle ligament and the midline of the gut become secured to the shell at the inverted point of the "V".
In the fully torted, advanced species of gastropod, suggested in stage 5, above, only one large gill remains as does only the left lobe of most other visceral organs, where organ lobes on the right side have largely disappeared or become vestigial. At the head end, water flow is propelled and directed by cilia that bring in nutrients past the gill. Excretory products are flushed out in a separately directed stream above the mouth.
Torsion is believed to be driven by an evolutionary adaptation that permitted the mollusc to more efficiently pull into its shell for protection. However, the most advanced orders of gastropod, e.g., the Opisthobranchs, cannot fully withdraw into their shell. They tend to lose full torsion, wherein the body plan reverts to the earlier bilateral symmetry characteristic of Neopilina galathea.
Archerd Shell Collection> Shell Classes > Torsion Illustration