Page:Popular Science Monthly Volume 20.djvu/467

Rh, may therefore greatly determine the character of its breathing organs. So it results that these organs, in the various groups of animals, are exceedingly diverse in form, structure, and position. They are much more diverse, indeed, than are the organs of any other function, 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 circulating fluids. And in origin, or morphologically, they are fundamentally 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 morphological character of the respiratory organs in the primary groups of the animal kingdom. While correct in the main, it does not cover all exceptional 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 vertebrates belong only to the alimentary canal. The description to follow will adhere to the order of the table: