Mollusca - Key Notes

Mollusca (Phylum Mollusca): Key Notes

  • General characteristics

    • Molluscs have soft un-segmented bodies.
    • Widely distributed in natural habitats.
    • Some are exclusively aquatic (e.g., mussels, octopus, oyster); others live in moist places (e.g., land snail).
    • They are triploblastic and display bilateral symmetry.
    • Possess true coelom.
    • Open-type circulatory system in which blood does not retain vessels; it directly bathes cells in tissue spaces (sinuses).
    • Body plan consists of three parts: head, visceral mass (contains organs of digestion, excretion, and reproduction), and foot (attached with visceral mass).
    • Mantle: an epithelial envelope around the visceral mass; the space between mantle and visceral mass is the mantle cavity.
    • In most molluscs, the outer surface of the mantle secretes a calcareous shell.
    • Radula: a rasping tongue-like organ present in all molluscs except Bivalvia.
    • Circulatory system: open in all molluscs except Cephalopoda (which have a closed system).
    • Heart: consists of a single ventricle and two auricles; expressed as 1 ventricle1\,\text{ventricle} and 2 auricles2\,\text{auricles}.
    • Digestive system: tube-like with gut having two openings—mouth and anus.
    • Excretory system: paired nephridia; wastes are gathered from sinuses and discharged into the coelom around the heart; nephridia open into the coelom; tiny cilia around nephridial openings move fluid from coelom into the nephridia; wastes are discharged into the mantle cavity and expelled.
    • Gills: used for gas exchange.
    • Nervous system: three pairs of interconnected ganglia present in the head, visceral mass, and foot; ganglia are interconnected by nerve cords.
    • Locomotion: movement is via a muscular foot.
    • Reproduction and life cycle: most molluscs are motile; some are sessile; most are unisexual.

  • Class Gastropoda

    • Examples: Slug; Garden snail; Helix aspersa (common garden snail).

  • Class Bivalvia

    • Examples: Freshwater mussel; Oyster.

  • Class Cephalopoda

    • Examples: Cuttlefish; Octopus.

  • Key anatomical terms to recognize on diagrams

    • Visceral mass
    • Mantle and mantle cavity
    • Shell (calcareous shell secreted by mantle in most molluscs)
    • Foot (muscular)
    • Radula (except in Bivalvia)
    • Gill (for gas exchange)
    • Coelom
    • Heart with 11 ventricle and 22 auricles
    • Nephridia (paired tubular excretory organs)
    • Nerve cords and ganglia (three pairs of interconnected ganglia)

  • Quick concept checks

    • Which class typically has a closed circulatory system? Cephalopoda.
    • What is the function of the mantle? To enclose the visceral mass and, in most species, secrete the calcareous shell; it forms the mantle cavity between itself and the visceral mass.
    • Where are wastes discharged from nephridia? Mantle cavity.
    • What is the number and arrangement of heart chambers in Mollusca (as described here)? 1 ventricle1\,\text{ventricle} and 2 auricles.2\,\text{auricles}.