RESPIRATION AND RELEASE OF ENERGY 



115 



every such organ the first essential is an increased surface, and the 

 different forms of gill merely represent various ways of attaining that end. 

 Among animals that use gills are fishes, some salamanders, crayfishes, 

 clams, some marine worms, and young stages of many insects. 



Lungs are internal cavities into 

 which air is drawn for absorption 

 of its oxygen. Notwithstanding 

 their internal location, lungs are 

 organs of external respiration, 

 since the bulk of the oxygen they 

 absorb is not used for energy 

 rel6ase in the cells of the lungs 

 themselves but is passed on to 

 other cells of the organism. 



The lung in lower amphibians 

 is a baglike organ with a large 

 central cavity (Fig. 94a); but 

 in higher amphibians it becomes more complex since its inner surface 

 is thrown up into corrugations with cross corrugations forming boxlike 

 spaces (b, c). These corrugations increase the respiratory surface. 

 In higher vertebrates the lung (d) is entirely subdivided into minute 

 air spaces which are in indirect connection with one another through 



Fig. 93. — External gills of the amphibian, 

 Epicrium glutinosum. {From Wiedersheim 

 after Sarasin.) 



BRONCHIOLE 

 WITH ALVEOLI 



Fig. 94. — Diagrams of types of lungs, a, amphibian lung with plain surface; b, amphib- 

 ian lung with low folds making simple alveoli; c, amphibian lung with higher folds which 

 are themselves folded making more numerous alveoli; d, human lung. 



large tubes, the bronchi, and their branches, the bronchioles. The 

 bronchi unite in a single large tube, the trachea, which is present in 

 the higher vertebrates, but absent in some of the lower forms, as 

 the frog. The trachea opens into the mouth through a slitlike glottis. 

 The trachea and bronchi have cartilage rings in their walls, so they 



