Annelid worms such as the earthworm are able to exchange gas

Annelid worms such as the earthworm are able to exchange gases using their skin. Why then do we see the development of complex gills in animals such as the clam and amphibian? (Why are gills adaptive?)

Solution

Annelid earthworms have moist epedermis due to which oxygen dissolved in the soil water can go into the earthworms body and hence exchange of gases takes place. Some amphibians and clams are devoid of moist epidermis and breathe through their gills.Moreover at some part of their lifetime they will have to live in aquatic enivironment and hence they need gills