Class Bivalvia Chapter 12

Bivalves 2 nd largest molluscan class 2 nd largest molluscan class Includes the clams, oysters, mussels, and scallops Includes the clams, oysters, mussels, and scallops Sheetlike mantle and shell with two valves cover Sheetlike mantle and shell with two valves cover Laterally compress animals Laterally compress animals Many are edibles, some form pearls Many are edibles, some form pearls

Con’t Most are filter feeders, valuable in removing bacteria from polluted water Most are filter feeders, valuable in removing bacteria from polluted water

Shells Valves – two convex halves Valves – two convex halves Margin of the shell is a proteinaceous hinge and tongue-groove modifications call teeth Margin of the shell is a proteinaceous hinge and tongue-groove modifications call teeth Umbo – the oldest part of the shell. Mantle lays down more shell in concentric lines of growth Umbo – the oldest part of the shell. Mantle lays down more shell in concentric lines of growth Shells form as a single structure as an embryo Shells form as a single structure as an embryo

Con’t Elastic hinge ligament opens the valves when certain muscles relax Elastic hinge ligament opens the valves when certain muscles relax Adductor muscles (dorsal side) close the shell Adductor muscles (dorsal side) close the shell The adductor muscles are the primary defense against predatory sea stars The adductor muscles are the primary defense against predatory sea stars

Con’t Mantle attaches to the shell around the adductor muscles Mantle attaches to the shell around the adductor muscles If sand or a parasite gets between the shell and mantle, the shell responds by secretes nacre to cover it, forming a pearl If sand or a parasite gets between the shell and mantle, the shell responds by secretes nacre to cover it, forming a pearl Pacific oysters form the highest quality pearls Pacific oysters form the highest quality pearls

Gas Exchange, Filter Feeding, and Digestion Gills are lamellae (folded sheets), one end attached to the foot, the other to the mantle Gills are lamellae (folded sheets), one end attached to the foot, the other to the mantle The ventral portion is inhalant region The ventral portion is inhalant region Dorsal region is the exhalant region Dorsal region is the exhalant region