CHAPTER XX 

 THE INTEGUMENT, TISSUES OF MECHANICAL PROTECTION 



THE integument consists of the exposed outer portions of the covering 

 epithelium of the body, added to, in nearly all cases, by certain connec- 

 tive tissue, muscle, and other cells that are located in its neighborhood. 

 By "exposed outer" is meant such portions as are directly exposed to 

 the medium in which the animal lives (air or water). The similarly 

 constructed coverings of many internal cavities into which water or 

 air are brought for respiratory or other purposes will be considered as 

 a form of integument and not further differentiated from the former in 

 this work except in so far as real differences exist between the two. 



On account of the superficial position of an integument, its functions 

 are most numerous. In the simplest and smallest organisms it performs 

 most of the functions of the body. In the larger and more complicated 

 creatures, many functions are still performed by portions of this surface 

 layer, but these portions are removed from the surface by invagination 

 to internal positions in the body, this happening in various degrees 

 according to the conditions. 



Although the surfaces used to perform the principal functions of the 

 body, in the majority of organisms, have been removed from the outer 

 integument, a number that cannot be performed internally have remained 

 in it, and in addition to these are found minor duplications of some of 

 those that have been removed to the inside. These latter have often 

 acquired some secondary use in the integument. 



With these functions as a chief basis for classification and with the 

 aid of ontogenetic origins, we may, for convenience, classify the integu- 

 mentary tissues as tissues of : 



A. Mechanical protection (and adornment). 



B. Offensive mechanical protection and production of poisons. 



C. Lubrication and cleansing. 



D. The production of attractive and repulsive odors. 



E. Adhesion and spinning. 



Of the functions of the integument, mechanical protection is one of 

 those that belongs peculiarly to it. We mean by the term a protection 

 against pressure, abrasion, and the entrance of needless or harmful fluids 



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