🐚 Phylum Mollusca – The Soft-bodied Coelomates of Protostomes

(Structure, Body Systems, and Habitat Adaptations)

PTB

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Literal Meaning:

• The word “Mollusca” is derived from Latin:

  • “Mollis” means soft.

• So, Mollusca literally means "soft-bodied organisms."

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General Characteristics:

• Body Texture: Soft, unsegmented body.

• Habitat:

  • Aquatic examples: Mussels, Octopus, Oyster.

  • Terrestrial (moist places): Land snail.

• Symmetry: Bilateral.

• Germ Layers: Triploblastic.

• Body Cavity: True coelomates.

• Developmental Group: Protostomes (blastopore forms mouth).

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Body Division:

• Mollusc body is divided into three main regions:

  1. Head – often bears sense organs and mouth.

  2. Visceral Mass – contains organs of digestion, excretion, and reproduction.

  3. Foot – a muscular structure for locomotion or attachment.

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Mantle and Shell:

• Mantle: A specialized epithelial layer that covers the visceral mass.

• Mantle Cavity: The space between the mantle and visceral mass.

• Shell: In most molluscs, the outer surface of the mantle secretes a calcareous shell for protection.

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Radula:

• All molluscs (except Bivalvia) have a rasping, tongue-like feeding structure called a radula.

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Circulatory System:

• Type: Mostly open circulatory system (except Cephalopods, which have a closed system).

• Heart Structure: One ventricle and two auricles.

• In an open system, blood flows into sinuses, bathing tissues directly.

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Digestive System:

• Tube-like gut with two openings: mouth and anus.

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Excretory System:

• Composed of paired nephridia (tubular structures).

• Function:

  • Wastes are collected from sinuses and released into the coelomic cavity around the heart.

  • Nephridia absorb wastes from the coelom using cilia and discharge them into the mantle cavity, from where they are expelled.

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Respiratory System:

• Gills (also called ctenidia) are used for gaseous exchange.

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Nervous System:

• Composed of three pairs of interconnected ganglia:

  1. Head ganglia

  2. Visceral ganglia

  3. Foot ganglia

• Ganglia are connected by nerve cords.

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Locomotion:

• Movement occurs via the muscular foot.

• Some molluscs (like oysters) are sessile (non-motile).

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Reproduction:

• Mostly unisexual (separate male and female organisms).

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