Mollusca - Classification

Classification

Opinions vary about the number of classes of molluscs; for example, the table below shows eight living classes, and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and solenogasters into one class, the Aplacophora. Two of the commonly recognized "classes" are known only from fossils.

Class	Major organisms	Described living species	Distribution
Caudofoveata	worm-like organisms	120	seabed 200–3,000 metres (660–9,800 ft)
Solenogastres	worm-like organisms	200	seabed 200–3,000 metres (660–9,800 ft)
Polyplacophora	chitons	1,000	rocky tidal zone and seabed
Monoplacophora	An ancient lineage of molluscs with cap-like shells	31	seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
Gastropoda	All the snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterfly	70,000	marine, freshwater, land
Cephalopoda	squid, octopus, cuttlefish, nautilus	900	marine
Bivalvia	clams, oysters, scallops, geoducks, mussels	20,000	marine, freshwater
Scaphopoda	tusk shells	500	marine 6–7,000 metres (20–23,000 ft)
Rostroconchia †	fossils; probable ancestors of bivalves	extinct	marine
Helcionelloida †	fossils; snail-like organisms such as Latouchella	extinct	marine

Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda.