546 



INTERRELATIONSHIPS OF ORGANISMS 



other huge algae among the plants; on the other it has permitted the 

 development of heavy armor and protective coverings, as in the starfishes 

 and shelled mollusks, without the necessity for powerful locomotor 

 organs. On land, the common earthworm and slug represent something 

 like the upper limits in size for a skeletonless, motile organism ; in the sea, 

 animals with as little mechanical support may attain a weight of a ton 



or more. 



The Medium and Locomotion. 

 Differences in the problems of 

 locomotion in water, on land, and 

 in air are even more marked than 

 those of mechanical support. The 

 motile aquatic animal has some- 

 thing of the same problems as the 

 lighter-than-air dirigible. Although 

 not lighter than the water (or very 

 little lighter), its weight is sup- 

 ported by the medium and its 

 locomotor organs have only to 

 propel it through the water. Since, 

 however, water is comparatively 

 dense, movement through it meets 

 with great resistance unless the 

 force required to displace the water 

 is largely balanced by the push 

 gained when the water in turn dis- 

 places the moving object. As a 

 consequence "streamlining" is 



Fig. 32.2. Support and locomotion in water. 

 Marine life around the wharf piles at Vine- 

 yard Haven, Mass. Large organisms such as 

 the jellyfish can exist without skeletal 

 structures because of the buoyancy of 

 water. The density of water makes stream- 

 lining a necessity for rapid motion, as illus- 

 trated by the fish and the squid. {Courtesy 

 American Museum of Natural History.) 



highly important. All motile aqua- 

 tic organisms, except those that simply drift and others that crawl 

 slowly (Fig. 32.3) are distinguished by some type of streamlined form. 



In terrestrial animals, on the other hand, locomotion involves not only 

 propulsion but the support of the body in a medium that supports less 

 than one-thirtieth of the body weight. Locomotor movements must 

 involve either the overcoming of a relatively great friction with the earth 

 (and this becomes increasingly difficult with each increment of size and 

 weight) or the lifting of the body and the maintaining of its equilibrium 

 as it is moved forward. All the larger and more motile terrestrial animals 

 have the greater part of the body weight accounted for in supporting 

 and locomotor appendages. Here streamlining is generally less important, 

 although it becomes a factor in the adaptation of certain terrestrial 

 animals to high rates of speed. Flight through the air demands still other 

 highly specialized adaptations, notably the development of great skeletal 



