Mollusca: Torsion in Gastropods and Cephalopods

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What is the definition of torsion in mollusks?

The twisting of the visceral mass relative to the foot and shell

What is the result of torsion in terms of body symmetry?

Asymmetry

What is the direction of the aperture in a twisted shell?

Facing downwards

What type of torsion is found in most gastropods?

Complete torsion

What is one of the adaptations of the twisted body plan in mollusks?

Protection of internal organs and head

What is one of the functions of the coiled shell in mollusks?

Efficient feeding and burrowing behaviors

Study Notes

Torsion in Mollusca

Torsion is a unique body plan characteristic found in mollusks, particularly in gastropods (snails and slugs) and some cephalopods (squids and octopuses).

Definition:

Torsion refers to the twisting of the visceral mass (containing the internal organs) relative to the foot and shell in mollusks.

Key Features:

  • 180° rotation: The visceral mass rotates 180° during embryonic development, resulting in the anus and mantle cavity being located above the head and foot.
  • Asymmetry: Torsion leads to asymmetry in the body, with the left side of the body being more developed than the right side.
  • Coiling: The shell coils in a spiral manner, with the aperture (opening) facing downwards.

Types of Torsion:

  • Complete torsion: Found in most gastropods, where the visceral mass rotates 180°.
  • Incomplete torsion: Found in some cephalopods, where the visceral mass rotates less than 180°.
  • Pseudo-torsion: Found in some mollusks, where the visceral mass does not rotate, but the body appears twisted due to other developmental processes.

Adaptations:

  • Protection: The twisted body plan provides protection for the internal organs and the head.
  • Feeding: The coiled shell allows for efficient feeding and burrowing behaviors.
  • Locomotion: The twisted body plan enables efficient movement and maneuverability.

Evolutionary Significance:

  • Convergent evolution: Torsion has evolved independently in different mollusk lineages, indicating its importance for survival and adaptation.
  • Diversification: Torsion has contributed to the diversification of mollusk body plans and the development of unique characteristics.

Torsion in Mollusca

  • Torsion is a unique body plan characteristic found in mollusks, particularly in gastropods and some cephalopods.

Definition and Process

  • Torsion refers to the twisting of the visceral mass relative to the foot and shell in mollusks.
  • During embryonic development, the visceral mass rotates 180°, resulting in the anus and mantle cavity being located above the head and foot.

Characteristics

  • Asymmetry: Torsion leads to asymmetry in the body, with the left side of the body being more developed than the right side.
  • Coiling: The shell coils in a spiral manner, with the aperture (opening) facing downwards.

Types of Torsion

  • Complete torsion: Found in most gastropods, where the visceral mass rotates 180°.
  • Incomplete torsion: Found in some cephalopods, where the visceral mass rotates less than 180°.
  • Pseudo-torsion: Found in some mollusks, where the visceral mass does not rotate, but the body appears twisted due to other developmental processes.

Adaptations and Functions

  • Protection: The twisted body plan provides protection for the internal organs and the head.
  • Feeding: The coiled shell allows for efficient feeding and burrowing behaviors.
  • Locomotion: The twisted body plan enables efficient movement and maneuverability.

Evolutionary Significance

  • Convergent evolution: Torsion has evolved independently in different mollusk lineages, indicating its importance for survival and adaptation.
  • Diversification: Torsion has contributed to the diversification of mollusk body plans and the development of unique characteristics.

Learn about torsion, a unique body plan characteristic found in mollusks, particularly in gastropods and some cephalopods. Discover how it affects the visceral mass and other body parts.

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