3 a Why is it useful for crabs that live on the sea shore to have both lungs and gills.

b If equal volumes of blood go through the lungs and gills of light-blue

soldier crabs, suggest the mean percentage of oxygen that would be found in

the blood having gone through only the gills. Explain your reasoning.

c What does this tell you about how efficient both lungs and gills are in these

crabs?

d Suggest three reasons for this difference.