102 How Animals Changed 



sanus) will remove practically all the oxygen from the water 

 around it, but a scup (Stenotomus) dies of suffocation when the 

 oxygen tension is still high (Hall, 1929). However, the rate of 

 oxygen consumption of a fish is lowered by a decrease in the oxygen 

 tension or an increase in the carbon dioxide tension of the sur- 

 rounding water; and a combination of these factors is more effec- 

 tive than either one alone (Powers dC Shipe, 1928) . Those fishes 

 which can rapidly change the alkali reserve of their blood survive 

 unfavorable gas conditions best. In worms also the rate of oxygen 

 consumption decreases lineally as the oxygen in the medium de- 

 creases (Hall, 1931). The rate of oxygen consumption varies in 

 invertebrates (Jordan, 1930; Duryee, 1932) and vertebrates (Hall, 

 1929) according to the amount of haemoglobin present in the 

 blood. MoUusks and crustaceans pass more easily to land than 

 vertebrates because the loading tension of haemocyanin is lower 

 than that of haemoglobin in corpuscles. In solution (oligochaetes) 

 haemoglobin has lower tension of loading than any other respira- 

 tory pigment (Carter, 1931). Among plethodontid salamanders 

 "the most terrestrial species are least active and have the lowest rate 

 of oxygen consumption (Evans, 1939) . As the habitats of estuarine 

 crabs "approached land there was an increase in oxygen consump- 

 tion" (Ayers, 1938). 



In many aquatic invertebrates the functioning of gills for respira- 

 tion is essential for life. For example, if the circulation of water 

 through the branchial chamber of a crab is prevented, death results 

 (Hogben 6C Zoond, 1930) . But there are some aquatic animals, 

 such as may-fly nymphs and caddis-fly larvae, which can live for 

 weeks or months after their gills have been removed (Morgan 6C 

 O'Neil, 1931; Morgan & Grierson, 1932), though their respiration 

 rate is slower. The land hermit crab Coenobita clypeatus Herbst 

 lives for months without gills if kept in air, but dies more quickly in 

 sea water without gills than with them (Borradaile, 1903; Pearse, 

 1932) . The so-called blood gills of certain insects are not always 



