Chapter 16: Molluscs

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Chapter 16 Molluscs
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Form and Function

• Mollusc body plan
• Head-foot
• Visceral mass

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Head-foot

• Contains the feeding, cephalic sensory and
  locomotor organs

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Visceral mass

• Contains digestive, circulatory, respiratory, and
  reproductive organs

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Mantle cavity

• Folds of skin form the mantle cavity
• Houses the gills or a lung
• In most molluscs, the mantle secretes a
  protective shell over the visceral mass

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Radula

• Rasping, tongue-like organ
• Scrapes off fine food particles from surfaces and
  conveys it to the digestive tract

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Foot

• Usually ventral
• Functions for attachment or movement
• Burrowers extend the foot into mud or sand, use
  the tip as an anchor, and draw forward

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Class Gastropoda

• Most diverse
• Contains snails, limpets, slugs, whelks, conches,
  periwinkles, sea slugs, sea hares and sea
  butterflies

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• Torsion is a twisting process that changes the
  larva from a bilateral to an asymmetrical form.

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Class Bivalvia

• Include mussels, clams, scallops, oysters, and
  shipworms
• Most are sedentary filter feeders
• Lack a head, radula or other aspects of
  cephalization

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Class Cephalopoda

• This class includes squids, octopuses,
  nautiluses, devilfish, and cuttlefish

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• All are marine predators.

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Form and Function

• The foot is in the head region and is modified
  for expelling water from the mantle cavity.

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Locomotion

• Cephalopods swim by forcefully expelling water
  through a ventral funnel or siphon.

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• It can control the direction and force of the
  water, thus determining its speed.

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Respiration

• Cephalopods have one pair of gills.

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• With higher oxygen demands, they have a muscular
  pumping system to keep water flowing through the
  mantle cavity.

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Nervous and Sensory Systems

• The cephalopod brain is the largest of any
  invertebrate.

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• Sense organs are well-developed eyes are
  complex (complete with cornea, lens and retina)

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• They can learn by reward and punishment, and by
  observation of others.

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Communication

• Cephalopods use chemical and visual signals.

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• Chromatophores are cells in the skin that contain
  pigment granules.
• Contractions of the muscle fibers surrounding the
  chromatophores causes the cells to expand and
  change the color pattern.

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• An ink sac secretes when the animal is alarmed,
  clouding the water to allow the animal to escape.