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Seashells belong to the phylum Mollusca, which contains about 100.000 species grouped into seven classes, according to their mouth, foot, gills, nervous system and shell: Aplacophora, Monoplacophora, Poliplacophora, Gastropoda, Scaphopoda, Bivalvia and Cephalopoda. About all classes include mollusks with shells; only the aplacophora and most member of cephalopoda, which are the most advanced mollusks, lack an external shell.
The color of the seashells is mostly produced by special pigments which deposite in the uppermost calcareous layer of the shell. Several mollusks have special chromogenic glands which can secrete, continuously or intermittently, colored pigments which stain the carbonatic ordinary white shell. The shape of many seashells show an astonishing variability: particularly, among the gastropods, the shell is generally spiral-shaped, and forms whorls that gradually increase in size; in other cases, a system of irregular spirals leads to the formation of rounded, globular shells, or the spirals are turred, fusiform or funel-shaped. The most beautiful and colorful seashells can be found in warm, shallow waters, most near coral reefs; on the contrary white and colorless shells are typical of arctic and deep-sea mollusks. As regard their distribution, seashells are widespread in about all the zoogeographical provinces, being most successfull in the temperate, tropical and subtropical areas and Caribbean. Beside living forms, more than 30.000 fossil seashell species, for the most part belonging to cephalopods (Ammonites), are known from Lower Cambrian to Cenozoic era.
Nautilidae
Patellidae
Cymatiidae
Trochidae
Bursidae
Strombidae
Terebridae
Turridae
Mitridae
Conidae
Epitoniidae
Cypraeidae
Muricidae
Thaididae
Coralliophilidae
Aporrhaidae
Haliotidae
Pleurotomaridae
Olividae
Cassididae
Triviidae
Cancellariidae
Volutidae
Architectonicidae
Tridacnidae
Glycymerididae
Glossidae
Veneridae
Pteriidae
Cardiidae
Mactridae
Pectinidae
Malleidae
Spondylidae
Ischnochitonidae
Mopaliidae
Dentaliidae
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