302 HISTORY OF THE HUMAN BODY 



through the general surface without the formation of localized 

 organs for the purpose, and in larger forms effective organs 

 of respiration are produced by the formation of external folds 

 or other outpushings of the integument. These are formed 

 in the embryo when the skin is still soft and thin and remain 

 in the adult state unaffected by the process of chitinization 

 which involves the surrounding integument. Such organs are 

 called gills, a general term for all aquatic respiratory organs. 

 These present numerous mechanical devices for increasing the 

 surface ; they may be in the form of single plates, sets of plates 

 placed parallel to one another, dendritic structures formed by 

 the repeated branching of simple diverticula, sets of parallel 

 tubes for the blood with interspaces for the water, and so on, 

 and are in most cases provided with accessory structures, 

 some for protection and others for producing a current of 

 water. 



In a terrestrial animal, on the other hand, the respiratory 

 system must be internal in order to secure the proper con- 

 ditions of moisture, and as all terrestrial animals are the 

 descendants of aquatic ones that succeeded in adapting them- 

 selves to the difficult environment of land, with its many dis- 

 advantages, it forms an interesting study in adaptation to 

 compare the respiratory system in each terrestrial group with 

 that of the animals which most nearly represent their aquatic 

 ancestors. In some cases the old respiratory organs are re- 

 tained by sinking them into deep recesses kept moist by glands, 

 in others they are discarded in favor of new ones, formed, per- 

 haps, by the transformation of some ready-to-hand cavity, 

 which is lined with blood vessels and made to communicate 

 with the exterior through some regulated outlet. Still another 

 principle is seen in the tracheal tubes of insects, which are 

 branching tubes lined with chitin and leading from a series 

 of external openings into the interior, ramifying all the inter- 

 nal organs. These, as shown by their development, are in 

 origin integumental folds, like the plate-like gills of their an- 

 cestors, which, as they develop, turn in instead of out, thus 

 satisfying the conditions of aerial respiration. 



