• 2) Class BIVALVIA, 2nd most diverse class (clams, oysters, scallops)

  • a) body plan modified for a burrowing, filter-feeding mode of life (body is laterally compressed & wholly enclosed inside mantle & shell; head & radula are absent)

  • b) shell composed of 2 valves joined by a flexible ligament; mantle edge is fused to the shell margin & linked to adductor muscles

  • c) foot highly modified for burrowing (except attached species); burrowing involves both shell and foot

  • d) 1 pair ctenidia; may be small & mainly respiratory (protobranch) or large and modified for filter feeding (lamellibranch: may be either filibranch or eulamellibranch type)

  • e) most produce byssal threads as larvae, some still do as adults

  • f) classification still hotly debated; 4 important subclasses:

  Subclass	Shell hinge teeth	Ctenidia	Adductor muscles
  Paleotaxodonta	'taxodont'	protobranch	isomyarian
  Pteriomorphia	none	filibranch	anisomyarian
  Heterodonta	'heterodont'	eulamellibranch	isomyarian
  Anomalodesmata	1 or none	eulamellibranch & septibranch	isomyarian

• 3) Class SCAPHOPODA, the 'tusk shells'

  • a) curious tubular shell; originates as two shells in larva that fuse

  • b) head reduced, but have a well-developed foot for burrowing

  • c) lack ctenidia but possess a radula; capture forams with captacula

• 1) Class CEPHALOPODA, 3rd major class (nautilus, squids, octopus)

  • a) includes most highly advanced, largest & fastest invertebrates; body plan modified for pelagic life

  • b) exclusively marine carnivores (eat fish, crustaceans, molluscs)

  • c) shell is reduced or absent; head & foot form tentacles & siphon

  • d) very sophisticated eyes and balance organs (statocysts)

  • e) 1 or 2 pair of ctenidia; have a true 'closed' circulatory system

  • f) mantle is muscular and mantle cavity large; jet-propulsion is achieved via antagonism of circular & radial mantle muscles

  • g) three subclasses (Nautiloidea, Ammonoidea-extinct, Coleoidea)

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