36 ZOOLOGY 



called, get along very well in running streams, where 

 fresh water, with its oxygen, is constantly passing 

 by. Brought into a laboratory and placed in a dish 

 of water, they die overnight of suffocation. In the 

 amphioxus and the lower vertebrates we find a notable 

 advance of structure, the development of gill arches. 1 

 The new plan enables the animal to cause a current of 

 water to flow through the gills, thereby giving all the 

 advantages of a running stream, even when the sur- 

 rounding water is quiet. The next great advance is 

 connected with the discovery of the land. Land life 

 implies the breathing of air, and yet it is not possible 

 to do this without some sort of moist chamber, in 

 which water will be constantly present and the delicate 

 blood vessels will not be in danger of desiccation. In 

 the insects and their relatives this end is gained by the 

 system of trachea, branching tubes connected with 

 small openings, or spiracles, on the sides of the body. 

 In the higher vertebrates, and also in most of the land 

 snails, the structure takes the form of one or more sacs, 

 known as lungs. Lungs in the vertebrates are in pairs, 

 and in the lowest forms are simple, moist cavities. 

 In warm-blooded animals, which have to maintain a 

 constant temperature, and are generally very active, 

 the need for oxygen is greatly increased. This cannot 

 be met by a corresponding increase in the size of the 

 lungs, which would assume the dimensions of balloons ; 

 so there arises instead an amazing complexity, which 

 gives an enormous increase of surface for absorption, 

 without any great addition to the external dimensions 

 of the organ. The spongelike tissue presents a vast 

 number of little cavities, into which air enters, and 

 through the walls of which gases pass. 



1 Young lungfishes and amphibians, and some adult amphibians, have 

 external gills. 



