244 STUDIES IN PHYSIOLOGY 



3. A COMPARATIVE STUDY OF THE SKIN 



The Skin of Invertebrates. In single-celled animals like 

 amoeba and paramecium, although the outer part of the 

 protoplasm is more dense than the rest, this layer cannot 

 properly be called a skin, for skin is an organ composed of 

 several tissues. The skin of the earthworm is an important 

 organ that serves for protection, respiration, excretion, and 

 sensation. We have already referred to the skeleton formed 

 by the coral and to the hard outer shells of insects, lobsters, 

 and clams. In reality all these outside skeletons are modi- 

 fications of the skin whereby these animals secure more 

 efficient protection against their enemies. The other func- 

 tions carried on by the human skin, namely, those of sensa- 

 tion, excretion of waste matters, and the regulation of heat, 

 cannot be performed through a hard shell. Hence for these 

 processes specially developed organs in other parts of the body 

 are necessary (see p. 252). 



The Skin of Amphibia. In our comparative study of 

 respiration we observed that the skin of tadpoles and frogs 

 is always soft and moist. These animals probably carry on 

 all four of the skin functions mentioned on p. 232, together 

 with the additional function of respiration. The outer cover- 

 ing of the toad is dry and warty, but none of the amphibia 

 are supplied with scales, feathers, or hairs, structures that 

 are characteristic of the other groups of vertebrates. At 

 certain times of the year the outer layers of epidermis are 

 shed by toads and frogs, and a new layer is then formed by 

 the living cells. In the human being the outer skin is con- 

 tinually worn off in minute bits, except after certain dis- 

 eases, like the measles and scarlet fever, when considerable 

 pieces come off at one time. 



The Skin of Fishes and Reptiles. Fishes and reptiles, in 

 most cases, have a characteristic outer covering composed of 

 scales. Fish scales are usually hardened portions of the epi- 

 dermis, projecting back ward, and overlapping like the shingles 



