166 SEA-SIDE STUDIES. 



plex organisms perform each of these functions by a 

 special apparatus of organs, yet these organs them- 

 selves are originally ^QYoio^^di from the envelope. We 

 may, ideally, reduce even a mammal to a cylindrical 

 envelope folded inwards at each end ; from the en- 

 folded skin 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 elemen- 

 tary 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 conjointly 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, ]3rehensile 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 substance between the inner and 

 the outer wall, and in this manner arise the various 

 mechanisms for respiration and reproduction. Such a 

 state of things is presented by the Actinia."* What 



* Draper : Human Physiology, 1856, p. 501. 



