Molluscs

• Molluscs

  • one of the largest animal phyla after Arthropoda
  • are coelomate lophotrochozoan protostomes
  • develop via spiral mosaic cleavage
  • make coelom by schizocoely
  • ancestral larva is a trocophore
• Molluscs

  • are distributed in marine, freshwater and terrestrial environments
  • probably the most malleable body plan in the animal kingdom with extremely dissimilar-looking organisms
  • includes clams, snails, octopuses
• Molluscs

  • 2nd largest phylim of over 160, 000 species described
  • around 128,000 are living
  • 35,000 are recorded as fossils
• Shell

  • most have shells consisting of CaCO3 set in protein matrix
• Shell

  • thin outer organic layer = secreted by edge of mantle
  • inner inorganic layer = secreted by entire mantle surface
  • innermost: thin calcareous layer
  • middle: thick calcareous layer
• Shell

  • microstructure can greatly differ among the members of different molluscan groups
• periostracum

  prismatic layer

  nacreous layer

  
• Periostracum

  • thin, outer layer
  • composition: conchiolin (resistant protein)
  • secretion: fold of the mantle edge
  • growth: growth occurs only at the margin of the shell
• Prismatic layer

  • thick middle layer
  • composition: CaCO3 laid down in protein matrix
  • secretion: glandular margin of the mantle
  • growth: increase in shell size occurs at the shell margin as the animal grows
• Nacreous layer

  • thin, innermost calcareous layer
  • composition: CaCO3 laid down in protein matrix
  • secretion: mantle surface
  • growth: becomes thicker during the life of the animal
• Mantle

  • sheath of skin that forms the dorsal body wall
  • a defining characteristic of molluscs
• Mantle Role:

  • protects soft parts
  • secretes the shell externally or internally
  • gives rise to respiratory organs
  • exposed surface serves for gas exchange
  • have sensory/tactile receptors for sampling the water (osphradium)
  • role varies substantially in different molluscan group
• Osphradium (mantle)

  • is generally located adjacent to the ctenidium
• Mantle Cavity

  • cavity lying between mantle and viscera
• Mantle Cavity Role:

  • serves as exit site for excretory, digestive, and reproductive systems
  • houses the comb-like gills (ctenidia)
  • creates jet propulsion in cephalopods
• Foot

  • a defining characteristic
  • adapted for:
  • locomotion
  • attachment to substratum
  • combination of function
• Foot

  • usually ventral, sole-like structure in which waves of contraction effect a creeping locomotion
  • mucus secretion for adhesion or slime track for gliding
• Foot Modifications

  • attachment disc (limpets)
  • hatchet foot (bivalves)
  • siphon (squid and octopuses)