Blood pigment in molluscs is usually
Molluscan blood pigment usually hemocyanin, copper-containing protein dissolved freely in plasma rather than enclosed in erythrocytes, forming blue color when oxygenated and colorless deoxygenated. Each functional unit contains two copper atoms coordinated by histidines reversibly binding one oxygen molecule, assembling into multi-decameric complexes with cooperative binding. Hemoglobin occurs in few freshwater bivalves and planorbid snails and some cephalopods as intracellular pigment, chlorocruorin primarily in some polychaete annelids, myoglobin muscular oxygen store. Hemocyanin predominate
Ref: Campbell Biology, 12th ed., Chapter 42: Molluscan Hemocyanin; NCERT Class 11, Chapter 7, Pigments