Exoskeleton is the hard protective and supportive frame work present on the outside of the body. Epidermal/ectodermal exoskeleton occurs in mammals (nails, hairs as in man. rabbits), birds as in fowl. Mesodermal exoskeleton occurs in fishes (scales as in Scoliodon) and some reptiles like crocodiles. Exoskeleton is absent in frog. Skin of frog is thin, moist, slimy and smooth.