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Torsion in Gastropoda

Torsion in Gastropoda. Dr. Inavolu S Chakrapani PRR & VS Govt. College Vidavalur. Adult Gastropod. Veliger Larva. Veliger Larva. General Organization of an Adult Gastropod. What is Torsion?. Rotation of visceral mass, mantle and shell 180 o with respect to head and foot.

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Torsion in Gastropoda

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  1. Torsion in Gastropoda Dr. Inavolu S Chakrapani PRR & VS Govt. College Vidavalur

  2. Adult Gastropod

  3. Veliger Larva

  4. Veliger Larva

  5. General Organization of an Adult Gastropod

  6. What is Torsion? • Rotation of visceral mass, mantle and shell 180o with respect to head and foot. • Occurs in all Gastropods during development • Brings mantle cavity and anus to the anterior position

  7. Whichfactors cause Torsion? • Contraction of larval retractor muscles • Differential growth of the visceral mass

  8. Before Torsion.. • Mantle cavity – posterior • Ctenidia and nephridia – posterior • Alimentary canal straight • Nervous system bilaterally symmetrical • Auricles located behind the ventricle • Visceral sac is dorsal • Shell planospiral

  9. The Mechanism of Torsion • One end of foot muscle is attached to the left side of the shell. Other end is attached to the left side of foot and head by fibres. • Its contraction causes an anti-clockwise90o rotation of the visceral mass and shell.

  10. The Mechanism of Torsion • Visceral mass grows more on the left hand side. • This causes further 90o rotation • Visceral mass bends in anterio-posterior sagittal plane.

  11. The Sequence of Torsion • Firstly, mantle cavity is displaced towards the right side and then to the anterior end. Head and foot remain fixed. • Digestive tract gets looped. Mouth and anus come nearer. • Saucer shaped visceral mass and shell become cone-shaped. These are finally coiled. • Ventral portion of the visceral mass and mantle rotates.

  12. The Sequence of Torsion • Dorsal mass is twisted. Right gill and right auricle remain. Corresponding parts on the left side are lost. • Before completion of the metamorphosis, there occurs a lateral torsion resulting in adult condition.

  13. Effects of Torsion • Mantle cavity opens just behind the head. • Anus and ctenidia come forward. Auricles lie in front of the ventricle. • Alimentary canal is twisted. Mouth and anus are approximated.

  14. Effects of Torsion • The long, uncoiled pleura-visceral nerve connectives are twisted to form an 8-shaped figure. Right connective becomes supra-intestinal and the left becomes infra-intestinal. • Visceral sac becomes ventral (endogastric). • Symmetry is lost.

  15. Significance of Torsion • Garstang’s View: Torsion is an adaptive feature and useful for veliger larva to protect the soft parts against predators. • C.M.Yonge’s View: Torsion is advantageous for ventilation of mantle cavity.

  16. Significance of Torsion • Morton’s View: Mantle cavity housing the head, sense organs and respiratory organs, if placed anteriorly, would be an advantage in aquatic environement. Hence torsion. • Ghiselin’s View: Primitive gastropods developed a conical shell on the dorsal surface. To maintain balance, body prolonged anteriorly. The weight of the shell is disadvantageous while crawling. Hence torsion evolved as a compensatory mechanism.

  17. Detorsion • Reversion of torsion is known as Detorsion • It is characteristic of Euthyneura. • Incomplete detorsion - Opisthobranchia and Pulmonata (Acteon, Bulla, etc.), - • Complete detorsion, - typical Opisthobranchia (Aplysia)

  18. Detorsion • In extreme cases, as in Pterotrachea, body elongates to become wormlike. • The mantle cavity, visceral hump, external shell and even ctenidia may be lost, as in Nudibranchia (Aeolts, Doris)

  19. The Process

  20. Thank you..

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