342 BIOLOGY. 



therefore mucus-walls. The external wall of the fishes 

 secretes an abundance of mucus, as does that also of 

 the worms, snails, and molluscs. 



1897. But an internal wall is still the more mucous 

 of the two, because it is darker and warmer. 



1898. At first the animal is content with the 

 antagonism of the walls ; especially so long as it remains 

 occluded in dark and deep water, or within other ani- 

 mals. Many intestinal worms, polyps, and even Acalephous 

 animals, are but simple sacs. 



1899. When the animal organization, however, ranl^s 

 upon a higher stage, light, or even air, operates more 

 upon its external wall, but upon its internal, water ; thus 

 the antagonism of the two walls is carried out to the 

 utmost degree. 



1900. Through the different, ay, opposed processes, 

 the two walls finally adopt another structure. The 

 external becomes denser and harder, on account of the 

 decomposition by light and dessication by air; the in- 

 ternal, however, retains its original structure and con- 

 sistence. Soft, aqueous, indifferent, and constantly 

 absorbent, it is only a viscous mucus. 



1901. In place of an integument of similar tissue 

 throughout, one will originate, whose external tissue is 

 dense and oxydized, but whose internal is loose or 

 spongy, and indifferent. The previously uniform integu- 

 ment will now separate or fall into two distinct layers ; 

 into a soft muco-cellular layer, and into a tough coriaceo- 

 cellular layer. 



1902. With the last attainable antagonism the layers 

 finally separate; two bladders or cysts, disunited from 

 each other, originate ; of these the internal is the mucous, 

 the external the coriaceous cyst. 



1903. Now the internal cyst alone is the intestine, the 

 external the cutis or skin. 



1904. Intestine and cutis belong to one formation, or 

 to the integument. They pass directly into each other 

 at the mouth and anus. Their structure also is wholly 

 similar. 



