Locomotion 125 



it must be low. . . . All these considerations lead to the conclusion 

 that the distribution of the blood is the dominant factor in metab- 

 olism and that the higher metabolism of warm-blooded animals 

 may be explained by the fact that in their case there is better distri- 

 bution of blood to the outer tissues and peripheries." In mammals 

 blood corpuscles are smaller than in fishes and amphibians. This 

 increases relative surface and oxygen-carrying capacity. 



Locomotion 



There is no essential difference between the locomotor organs of 

 marine and fresh-water animals, but those of animals which have 

 left aquatic habitats to take up life on land have often been obliged 

 to undergo considerable modification. In transferring its activities 

 from water to land an animal enters a rarer medium and thus has 

 an opportuntiy to move with a greater speed, but it cannot do this 

 if it does not have sturdy locomotor organs which are able to sup- 

 port its body against the greater pull of gravity which results when 

 the buoyancy of water is lost. Seals and whales are powerful and 

 agile swimmers, but on land are awkward or incapable of locomo- 

 tion. 



The chief invaders of the land are: (1) animals which creep on 

 slimy muscular surfaces, such as flatworms and snails; (2) those 

 with hard exoskeletons, the arthropods; (3) and those with endo- 

 skeletons, the vertebrates. Terrestrial worms and mollusks have re- 

 mained subterranean or are confined largely to moist situations. 

 None has attained speed, which Jehu (1923) maintains is a primary 

 quality of a typically terrestrial animal. It is the arthropods and 

 tetrapod vertebrates which are today the dominant land animals. 



Arthropods in any habitat are covered with a chitinous exoskele- 

 ton, which is also often impregnated with mineral salts and thus 

 strengthened. Their jointed legs are suited for locomotion of various 

 types. They may be flattened and provided with long bristles, and 

 thus serve as admirable organs for swimming, or they may be pro- 



