Gills 



Some water insects breathe 

 air from above the water sur- 

 face through special open- 

 ings into the tracheae. The 

 hellgrammite and a fewothers 

 breathe through leathery gills, 

 which expose relatively large 

 surfaces to the water 



A WATER-BREATHING INSECT 



air at the surface of the water through special breathing tubes. The "hell- 

 grammite", the larval stage of the Mayfly and of the Dobson fly, has pro- 

 jecting gills, through which air is absorbed from the water. 



Life without Air A few species generate energy without a supply of 

 oxygen. In yeast and in certain other simple plants, ferments, or enzymes, 

 bring about the breakdown of carbohydrates into simpler compounds, as 

 alcohol and carbon dioxide, in the absence of oxygen. Such organisms are 

 called anaerobic^ that is, living without air. The release of energy from com- 

 plex chemical compounds without oxidation may be likened to the release 

 of energy that results from the collapse of a structure when a particular 

 small detail is disturbed. 



Breathing in the Vertebrates^ All the backboned animals, except the 

 fishes and the young stages of amphibians, breathe by means of lungs. In 

 the fishes, water with oxygen in solution is taken into the mouth. But 



Water inside the clam's shell 

 is kept in constant circulation 

 by the vibration of cilia which 

 cover the whole surface of 

 the body, the lining of the 

 mantle, and the surfaces of 

 the gills. The water also 

 passes through tiny openings 

 in the gills themselves. As the 

 water passes over the gill 

 surfaces, gas-exchange takes 

 place between the flowing 

 water and the blood circulat- 

 ing Inside the gills. Water 

 comes into the mantle cavity 

 and is discharged again 

 through the siphon 



Gills 



Mouth 



HOW THE CLAM BREATHES 



iSee No. 9, p. 213. 

 209 



