ADDITIONAL PACTS ABOUT THE CRUSTACEANS. 73 



Additional Facts about the Crustaceans. 



The crawfish introduces us to a different mode of 

 breathing ; that is, by means of gills. Breathing, or 

 respiration, consists essentially of an exchange of 

 gases. The tissues of the body need to be furnished 

 with oxygen. This oxygen must come indirectly 

 from the air. When oxygen reaches the tissues of 

 the body, some of it is used in doing work or in gener- 

 ating heat. The result in either case is the produc- 

 tion of carbon dioxide, which must be removed from 

 the, body. In the crawfish, the blood carries the 

 carbon dioxide from the tissues, where it is generated, 

 to the gills. The water which flows over the gills 

 contains oxygen, which it has absorbed from the air. 

 The blood carrying carbon dioxide flows into the gill 

 filaments, where it is separated from the oxygen con- 

 tained in the water by only a thin, moist membrane, 

 which is the outer layer of the gill filament. Under 

 these conditions, the carbon-dioxide carried by the 

 blood leaves the blood and passes through the gill 

 membrane into the water, and the oxygen in the 

 water leaves the water and passes through the gill 

 membrane into the blood. 



In order that this exchange of gases may take 

 place, the gill membrane must be kept moist. A 

 crawfish can live out of water as long as its gills are 

 moist. This will enable us to see the advantage of 

 the gill covering. It prevents the rapid drying out 

 of the gill and enables the crawfish to live some time 

 out of water. If necessary, the crawfish can travel 

 overland from pond to pond. 



There must be oxygen in the water. Water that 

 has been boiled has had all the absorbed oxygen 

 driven off. So crawfishes cannot live in boiled water 

 unless it has absorbed oxygen since the boiling. 

 Water that has given up its oxygen to the gills must 



