Gastropods are thought to have arisen from a monoplacophoran (a single bilaterally shelled mollusc) ancestor who underwent torsion, the twisting of the visceral mass above the foot by 180°(Barnes, 1963). This was followed by the elongation of the shell and visceral mass for retreat when threatened and planispiral coiling and later conispiral coiling of the shell. These characteristics can be easily seen in the group of gastropods known as the Prosobranchs while more derived gastropods such as the Opisthobranchs have undergone partial detorsion (Barnes, 1963). Opisthobranchs are characterised by their secondarily bilaterally symmetrical (euthyneurous) nervous system, which replaced the twisted (streptoneurous) prosobranch nervous system, and their tendency to reduce/lose their shell. The detorsion associated with the euthyneuran nervous system highly concentrates and cephalises the ganglia. Opisthobranchs share this characteristic with the pulmonates with which they can sometimes be grouped with and known as Euthyneurans and the sister group of Streptoneurons (Barnes, 1963). Detorsion and reduction/loss of their shell allowed Opisthobranchs to exploit habitats poorly or not at all exploited by prosobranchs.