Molluscs

• Mollusca is the largest phylum of animals, with over 100,000 species.
• The mollusc body plan consists of three main parts: head (with eyes), visceral mass (digestive system), and foot (for locomotion).
• Molluscs have a soft body covered by an external shell or internal organs that secrete a protective layer called mantle.
• torsion stage 1


  90 counter clockwise turn, second larval stage, retraction of foot muscles
• stage 2 of torsion

  90 counter clockwise turn, differential tissue growth
• torsion


  180 turn from posterior to anterior; visceral mass, mantle cavity and shell
• disadvantages to torsion


  tanglement of digestive/nervous system, waste product over gills, issues with generating internal water flow
• advantages of torsion


  improved balance and foot positioning aids locomotion, protect head via retraction, prevents fouling (reduced gill interaction with environment)
• haliotis and diodora


  water enters anteriorly, passes over gills, then, anus, then nephridipores and exits. waste does not pass over gills
• possobranchs


  lost right gill, nephridipore and other associated paired organs. unidirectional flow of water entering from left exiting from right. optimises gas exchange despite loss of one gill since no waste passing over
• opisthobranchs


  subclass, detorted, partial/full reverse of torsion, more symmetrical body plan, advantages in locomotion and feeding