1) Class APLACOPHORA (odd, worm-like molluscs) ~300 marine spp
a) small (usually <10 mm long), mostly deep-sea predators; some live intertwined among hydroids & eat polyps, others inhabit burrows in the sediment & eat micro-infauna
b) shell & sometimes gills absent; girdle sometimes bears spicules
c) head & foot reduced or absent
d) diffuse, ladder-like nervous system
e) radula and ctenidial form link them to molluscs
f) most are hermaphroditic; some species possess a trochophore larva but others exhibit direct development
2) Class POLYPLACOPHORA (chitons) ~800 marine spp; many fossils
a) generally shallow-water or intertidal grazers on hard bottom
b) odd 8-plated shell is surrounded by an extensive 'girdle' (mantle)
c) 8 pairs of pedal retractor muscles connect the foot to the plates
d) multiple ctenidia (6 - 90 pr.) lie in lateral grooves of mantle cavity
e) 1 pair of large nephridia & 1 pair of gonads (embryonically)
f) diffuse, ladder-like nervous system
g) sexes are separate; free-spawning species have a trochophore larva that develops gradually into a juvenile chiton
3) Class MONOPLACOPHORA (primitive limpet-like molluscs with an extensive fossil record) only 20 living marine spp.
a) deep-sea grazers on hard bottom; believed to be extinct for over 350 million years (since Devonian!) until re-discovered in 1952
b) single cap-shaped shell covers the body, muscular creeping foot
c) their anatomy hints at a segmental ancestry because they have: 8 pairs of pedal retractor muscles, 5-6 pairs of gills, 6-7 pairs of nephridia, 2 pairs of gonads, 10 'rungs' in nervous system
d) fossil evidence and comparative anatomy shows they are sister group to all higher Mollusca (='Conchifera', all share loss of diplosome)
e) early embryology and larval forms (if any) remain unknown!
4) SEGMENTAL ANCESTRY
Eight-fold repetition of parts in adult Monoplacophora, chitons, and fossil bivalves, and in larval chitons and Aplacophora, suggests ancestral mollusc may have had a weak eight-fold segmentation