: The nervous system is simplified, consisting of three pairs of ganglia. Ecological and Economic Importance
: They possess an open circulatory system with a heart and vessels. : The nervous system is simplified, consisting of
: The body is laterally compressed and enclosed in a bilateral shell. This shell consists of three layers: an outer organic layer (periostracum), a middle calcareous layer, and an inner pearly layer (nacre). : The nervous system is simplified
: By filtering vast amounts of water, bivalves like mussels and oysters act as natural water purifiers in ecosystems. a middle calcareous layer