According to research, the phylum Mollusca can be divided into eight classes. Their classification and morphological characteristics are as follows:

1. Polyplacophora
   Chitons are representative of this class. Their bodies are flattened and oval-shaped, with the mantle exposed on the dorsal surface. The back bears eight shell plates arranged in overlapping rows. They lack tentacles and eyes. Several pairs of gills are arranged in lateral grooves along the sides of the body. A radula is present in the mouth, and the nervous system is ladder-like. All are marine, inhabiting rocky shores and feeding on algae. About ten species are recorded in Taiwan.
2. Caudofoveata and Solenogastres
   The body is elongated, with a mouth at the anterior end but no eyes or tentacles. A longitudinal groove runs along the ventral side, and the dorsal surface is covered with calcareous scales or spicules. The radula is small or reduced, and gills are absent; respiration occurs through the body surface. These have not yet been recorded in Taiwan.
3. Monoplacophora
   These possess a single, cap-shaped shell. They lack eyes and tentacles but have five pairs of gills. A radula is present in the mouth, and the nervous system forms a ring. Only three living species are known, all inhabiting deep-sea environments. They have not been recorded in Taiwan.
4. Gastropoda
   The body is divided into a head, visceral mass, and muscular foot. The head is well developed with eyes and tentacles. A well-developed radula is present in the mouth, and the muscular foot is adapted for crawling. The mantle secretes a shell, usually spirally coiled, which may be dextral (right-handed) or sinistral (left-handed). Many species possess an operculum to close the shell opening. Gastropods are highly diverse, including carnivorous, herbivorous, and omnivorous species. They occur in marine, freshwater, and terrestrial environments. Approximately 1,700 species are recorded in Taiwan.
5. Scaphopoda
   These have tubular or tusk-shaped shells that are slightly curved and open at both ends. The muscular foot extends from the wider opening and is used for burrowing into sand or mud. A radula is present, but eyes and tentacles are absent, and gills are reduced. Species diversity is low. All are marine and inhabit sandy or muddy substrates. Commonly known as tusk shells or “elephant tusk shells,” the exact number of species in Taiwan is uncertain.
6. Bivalvia
   Also known as Pelecypoda or Lamellibranchia, bivalves have laterally compressed bodies enclosed within two hinged shells. They lack a distinct head, eyes, and tentacles. Respiration and filter-feeding occur through siphons. The hatchet-shaped foot extends outside the shell for movement, and shell opening and closing are controlled by the antagonistic action of adductor muscles and ligaments. Bivalves are highly diverse, including edible marine species such as clams, oysters, razor clams, and tongue shells; freshwater species include mussels and clams. About 600 species are recorded in Taiwan.
7. Cephalopoda
   Cephalopods include octopuses, cuttlefish, squids, neritic squids, and nautiluses. They possess well-developed heads and eyes, and most exhibit agile jet propulsion. All are carnivorous. The external shell is reduced in most species, such as commercially important cuttlefish and squids. The only living cephalopods with a true external shell are nautiluses, which inhabit deep waters of the southwestern Pacific.