Besides the three well known classes the mollusc phylum contains five further less well known classes.
Generally the mollusc phylum can be divided into two subphyla, the Aculifera (spiny molluscs) and the Conchifera (shell bearing molluscs).
The subphylum Aculifera comprises groups of all those molluscs, that originally do not produce a complete shell. Instead they are protected by a hard exterior skin called a cuticula. This cuticula additionally is protected by calcareous spines (spiculi).
Two classes belong to the Aculifera:
Aculifera
Aplacophora.
Chaetodermomorpha (= Caudofoveata)
Neomeniomorpha (= Solenogastres)
Polyplacophora
(Chitons).
The Conchifera subphylum comprises the groups of molluscs bearing a primarily complete shell. This shell among molluscs of various groups may be very different. Mussels' (Bivalvia) shells are bipartite, while snails' (Gastropoda) shells are coiled in an asymmetric spiral. Squids' (Cephalopoda) shells usually are reduced secondarily. Among the Conchifera there are also the ancient molluscs of the Monoplacophora group, which is present on earth only in one recent order, the Tryblidia. This order contains the living fossil Neopilina galatheae, that was found in 1953 during an exploration in deep waters of the coast of Southern America.
Conchifera
Monoplacophora
Tryblidia
(Neopilina)
Visceroconcha
Gastropoda
Cephalopoda
Loboconcha
Scaphopoda ("Tusk shells")
Bivalvia
The division into Visceroconcha containing the classes Gastropoda and Cephalopoda, as well as into Loboconcha containing the classes Scaphopoda and Bivalvia is to make clear a closer relationship between the two respective classes than between these and the three other, which especially shows in the type of shell production.
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