XXIV. SKIN STRUCTURE AND EXCRETION 

 IN THE LOWER ANIMALS.' 



In comparing the skin of man with that of the lower 

 . animals we shall have to modify our conception of the 

 word skin. In man we used the word to describe a two- 

 layered system of tissues whose function was fourfold : pro- 

 tection of underlying parts, excretion of wastes, the end 

 organ of various sense nerves, and the regulator of the 

 body temperatm-e. When we speak of the skin of the 

 lower forms, we have in mind only one characteristic of 

 the human skin, namely, a protective layer of some kind. 

 In the consideration of the different groups of animals it 

 will be of interest to note the manner in which the body 

 coverings of the various forms approximate to the complex 

 structure of man. 



Protozoans. — Since the protozoan consists of only one 

 cell, it can evidently have no skin in the sense of a tissue 

 structure. Many protozoans, however, do develop a wall 

 or outer covering for their protoplasm which in some re- 

 spects fulfils the functions of the skin of higher forms. For 

 example, in the thin membrane or cuticle, wMch is secreted 

 by the ectoplasm of the paramoecium, we have a layer 

 which protects the contents of the cell, and at the same 

 time permits the exchange of oxygen and carbon dioxide 

 to take place through it. In permitting the escape of 

 carbon dioxide it is as truly a means of excretion as the 

 thicker membranes of other forms. 



'See Eootnote, p. 146, Chapter X. 

 374 



