• 1) Class APLACOPHORA, small, odd, marine, worm-like molluscs
  
  a) shell & sometimes gills absent, head & foot reduced or absent
  b) radula and ctenidial form link them to molluscs

• 2) Class POLYPLACOPHORA (chitons) marine grazers on hard bottom
  
  a) an unusual 8-plated shell is surrounded by an extensive 'girdle'
  b) 8 pairs of pedal retractor muscles connect the foot to the plates
  c) multiple ctenidia lie in lateral grooves of the mantle cavity

• 3) Class MONOPLACOPHORA, primitive marine limpet-like molluscs, deep-sea grazers on hard bottoms
  
  a) single cap-shaped shell covers the body, muscular creeping foot
  b) unusual because have 8 pr. pedal retractor muscles, 5-6 pr. gills, 6-7 pr. nephridia, 2 pr. gonads, 10 'rungs' in nervous system
  c) good fossil evidence & comparative anatomy shows they are sister group to all higher Mollusca; together they form the 'Conchifera'

• 4) Eight-fold repetition of parts in adult Monoplacophora, chitons, & fossil bivalves, and in larval chitons & Aplacophora, suggests ancestral mollusc may have had weak eight-fold segmentation

• 5) Class GASTROPODA (snails & slugs) the most diverse class; found in the widest range of habitats- marine, freshwater, terrestrial
  
  a) have 1, usually coiled shell
  b) have a large, creeping foot; most move using muscular waves
  c) united by a bizarre developmental process: torsion (twisting of posterior end over head; adaptive significance remains a puzzle)
  d) primitively were rasping grazers, but feeding ecology is diverse
  e) divided into 3 subclasses:

  - Prosobranchia (marine snails) large shell, anterior mantle cavity, 1 or 2 ctenidia- some unipectinate, operculum, separate sexes

  - Opisthobranchia (sea slugs) shell & mantle cavity reduced or absent; ctenidia & operculum absent in most; hermaphroditic

  - Pulmonata (land snails & slugs) shell present or absent; mantle cavity is a lung; ctenidia & operculum absent in all; hermaphroditic; ancestor was likely terrestrial, not marine

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