Passage of Medium through Organism 35 



a herd of antelope in an old Ford and then release the cheetah for 

 the last short dash. 



In the aquatic environment fish of the mackerel tribe are the fastest 

 swimmers and attain speeds as great as 48 km per hr (30 miles per 

 hr). The flying fish has been reported to attain 56 km per hr just 

 before its take-off. Anyone who has watched a flying fish, however, 

 will remember that in the last moments before the fish leaves the 

 water most of its body is in the air, with only the tail scuUing violently 

 in the surface like an outboard motor. 



Even the method of propulsion is controlled to a large extent by 

 the elemental difi^erence in the nature of the air and water media. 

 Since the density, viscosity, and inertia of air are so low, most animals 

 cannot use the air alone for propulsion but must obtain a purchase 

 on the earth's surface. Only birds, insects, and a few other animals 

 can propel themselves wholly in air. In the aquatic environment, on 

 the other hand, the majority of animals swim in the free water, and 

 those for which speed is important do not use the substratum for ef- 

 fective locomotion. The lobster, for example, pokes around on its 

 walking legs, but to make a sudden dash it uses swift strokes of its 

 tail, letting its legs leave the bottom entirely. Because of the rela- 

 tively great inertia of the water medium, some animals, such as jelly 

 fish and scallops, can propel themselves in one direction by pumping 

 water in the opposite direction— an early version of "jet propulsion." 

 The squid can dart backwards with remarkable rapidity by ejecting 

 water from its siphon. 



Passage of Medium through Organism 



Sometimes the medium must move through the organism instead 

 of, or in addition to, the movement of the organism through the 

 medium. Because of the great mobility of air, this medium can move 

 in and out of the cavities of an animal or plant with relative ease, but 

 water does not circulate as freely. Special adaptations are required 

 to carry water to the tops of trees, including root pressure, transpira- 

 tion, and the tensile strength of fine water columns. The movement 

 of the water through the tracheids of the plant is relatively slow. 

 Even in aquatic plants direct water exchange by osmosis or colloidal 

 imbibition requires considerable physical force. 



Although large expenditure of work is necessary, some aquatic ani- 

 mals do succeed in causing water to flow through relatively simple 

 respiratory or feeding chambers. But water could not possibly be 

 pumped into and out of a finely branched system of tubules like the 



