1. Their bodies are soft, fleshy and unsegmented.
2. They are cylomates, asymmetrical or bilaterally symmetrical and have a conspicuous head.
3. At the base of the body are muscular fleshy legs.
4. The body is covered by a thin covering called mantle. Calcareous material secreted from the mantle forms the shell.
5. Respiration occurs by gills and mantle membrane. Pulmonary sacs are developed in terrestrials.
6. The digestive tract consists of sand-tongues or radulas made of chitin.
7. The body cavity is very short and turned into a hemocele. Circulatory system is semi-free type.
8. The alimentary canal is straight, curved or U-shaped.
9. Blood contains hemocyanin (meat and copper) and amoebocytes.
10. Siloam is limited only to the sides of the heart, the genital cavity and the kidneys.
11. A unisexual or bisexual animal. They have sex.
12. The life cycle consists of trochophore, velizar or gochidium larvae.
13. They live in sea salt water, fresh water, ditches or pits.