IMPORTANCE OF THE ANIMAL ENVELOPE. 157 



as the lowest, the envelope is of eminent importance, its 

 predominance bearing a jn-ecise ratio to the simplicity of the 

 organism. The simplest organisms breathe, exhale, secrete, 

 absorb, and reproduce by their envelopes alone ; and if the 

 more complex organisms perform each of these functions by 

 a special apparatus of organs, yet these organs themselves 

 are originally developed from the envelope. We may, 

 ideally, reduce even a mammal to a cylindrical envelope 

 folded inwards at each end ; from the enfolded sldn are 

 developed all nutritive and reproductive organs, while the 

 nervous system and its osseous sheath are developed in the 

 space between the outer and inner walls of the envelope. 



" We may, in an ideal manner," says Professor Draper, 

 " conceive the production of the more elementary animal 

 forms, as arising from a simple sac or bag, which, furnishing 

 a starting-point, exhibits its first acquirement of localisation 

 of function, by the doubling of one half into the other, 

 thereby giving rise to a cup or pocket-shape form, so that 

 respiration and digestion, which were confusedly and con- 

 jointly carried forward upon the same surface, are now 

 parted from each other, the outside of the cup being devoted 

 to the one, and the inside to the other. Increased endow- 

 ments are obtained by crimping or dividing the edge of the 

 cup, prehensile organs of less or greater length and power 

 arising thereby ; and this in reality is the structure of the 

 Hydra. Another advance is made by the preparation of 

 new and complicated structures, fashioned out in the sub- 

 stance between the inner and the outer wall, and in this 

 manner arise the various mechanisms for respiration and 



