HOW ANIMALS BREATHE. 



45i 



stances, may therefore greatly determine the character of its breathing- 

 organs. So it results that these organs, in the various groups of ani- 

 mals, are exceedingly diverse in form, structure, and position. They 

 are much more diverse, indeed, than are the organs of any other func- 

 tion, and any classification now possible is quite arbitrary. 



In purpose, however, or physiologically, all breathing-organs are 

 simply an expansion of surface for the more rapid aeration of the cir- 

 culating fluids. And in origin, or morphologically ', they are funda- 

 mentally a modification or development of the skin ; being developed 

 either primarily, i. e., directly from the skin itself, or secondarily, i. e., 

 from the alimentary canal, which is itself so derived. 



The following tabular classification shows the general morphologi- 

 cal character of the respiratory organs in the primary groups of the 

 animal kingdom. While correct in the main, it does not cover all ex- 

 ceptional cases, but fairly exhibits the development of the organs and 

 the " differentiation " of the function from the lowest to -the highest 

 animals. Except in the vertebrates no regard in this table is paid to 

 any distinction between air and water breathing. 



In explanation : A stands opposite the mode of respiration which 

 is characteristic of the group beneath which the letter occurs ; and the 

 diminishing degrees in importance of the various methods are indicated 

 by the small letters according to the alphabetical order : but of course 

 the same letter is not of equal value for different groups of animals. 



The reader will observe the common use throughout the whole 

 animal kingdom of the skin unmodified. Also that the special organs 

 of the invertebrates, which number more than nine tenths of the whole, 

 are quite limited to modifications of the skin ; while those of verte- 

 brates belong only to the alimentary canal. The description to follow 

 will adhere to the order of the table : 



RESPIRATION BY 



*-* cz 

 o ' 



H A3 



a g 

 < c 



2 



2 "o 



A 



b 



Vertebrata. 



J 

 < 

 a 

 S 



4 



