There are five classes of animals with the back-bone (vertebrates) viz. Pisces i.e. the fish, amphibia like the frog, reptilia e.g. snakes, aves i.e. the birds and mammalia including us, humans.
Before the chordates came the proto-chordates. There are two groups of them, the - urochordata and cephalochordata. Urochordata skin has cellulose in it. Yes, cellulose ! Something you always associated with plants. Please take a look at the sea-squirts , the animals with cellulose in their skins- http://www.ucmp.berkeley.edu/chordata/ur...
Keratin is a fibrous scleroprotein that occurs in the outer layer of human and other animals’ skin in horny appendages such as hair, feathers, nails and hooves. No other animals are known to have cellulose, a polysaccharide, which is typically present in plants.
Lampreys, belong to the group, cyclostomata ( round mouth ) some of the most primitive vertebrates. Their skin is slimy and does not have any scales.
Sharks are the cartilaginous fish. They have the dermis, and the epidermis and small scales of a type called placoid ( platelike) scales. The placoid scales can form teeth.
Sharks can produce as many number of teeth as they fall off and as many times as they fall off. (Had there been dentist sharks they would have no work at all.)
Bony fish like the salmon, labeo, tuna, sardins, pomphrets have in their skin a variety of scales – cosmoid, ctenoid, ganoid – though all the fish do not have scales. Exceptional fish who do not have any scales are –the clingfishes (family Gobiesocidae). Their bodies are protected by a thick layer of mucous.
And yes, in some species of fishes e.g. flatfishes like, soles, type of scales may differ in two sexes. The males have ctenoid scales and the females have cycloid scales.
Please visit - http://www.amonline.net.au/fishes/studen... - an Australian museum site to know more about fish.
As the first amphibians started venturing onto the land they used the scaly exterior of the fish to prevent drying out of the body surface. Modern day frogs or toads have no scales and have a thin moist, respiratory skin.They must keep their skin fairly moist and can not, therefore, go very far away from waterbodies. Amphibia have mucus and poison glands in their skin. the poison glands only produce an irritant and handling a toad may give rashes on account of this so called poison.
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