256 Oxygen and Carbon Dioxide 



medium. Sessile animals cause water to flow through their respira- 

 tory chambers or wave their respiratory organs about in the surround- 

 ing water. The echiuroid worms keep a current of water flowing 

 through their burrows in the sand when the tide is in by rhythmic 

 contraction of their bodies, but reduced metabolism allows them to 

 withstand periods of 18 hours when their tide flats are out of water. 

 More active animals, such as most fish and higher crustaceans, possess 

 special mechanisms for pumping water continuously over their gills. 

 Certain fish, however, reverse the process and produce a flow over 

 the gills by swimming rapidly through the water with their mouths 

 open. Some species, as, for example, the mackerel, have come to 

 rely entirely on this latter method, and in the course of evolution the 

 gills and opercular muscles have become reduced, with the result 

 that these fish cannot obtain sufficient oxygen while remaining sta- 

 tionary. If a mackerel is confined in a small space, or prevented from 

 swimming rapidly, it will die of asphixiation even though the sur- 

 rounding water is saturated with oxygen ( Hall, 1930 ) . 



In view of the foregoing one might expect that in those aquatic 

 habitats in which the water is not saturated with oxygen, the lack of 

 this vital substance would immediately have serious consequences. 

 However, many aerobic aquatic organisms are quite able to survive 

 with concentrations of oxygen much below normal pressures. The 

 metabolism of some species is so low that a small oxygen supply is 

 sufficient. Certain species possess special respiratory pigments or 

 other physiological adaptations that aid in tlie absorption of oxygen 

 at very low partial pressures. Chironomid larvae, inhabiting the 

 muddy bottoms of ponds, are furnished with a type of hemoglobin 

 that becomes 95 per cent saturated under a partial pressure of oxygen 

 of only 10 mm Hg, whereas mammalian hemoglobin is less than 1 per 

 cent saturated at this pressure. These insects are also capable of a 

 remarkably simple anaerobic metabolism in which the lactic acid 

 formed from glycogen breakdown is excreted, thus eliminating this 

 toxic compound rather than oxidizing it as most animals do. 



The relation of oxygen consumption by animals to the tension of 

 oxygen in the environment follows two general patterns: (1) the non- 

 regulatory type in which consumption is highly dependent upon ten- 

 sion, as seen in certain annelids and arthropods ( e.g.. Nereis, Homarus, 

 Limuhis, and Callinectes), and (2) the regulatory type in which con- 

 sumption is independent of oxygen pressure over a wide range, as 

 seen in certain crustaceans and mollusks (e.g., Astacus, Carcinus, 

 Aplysia, and Eledone). In the latter type the harmful effects of in- 

 sufficient oxygen supply appear rather suddenly when the oxygen 



