100 



THE RISE OF ANIMAL LIFE 



Fig. 5-10. A typical fresh-water pond. The association of plants and animals in such an environment is extremely 



complex. 



possesses unique eyes which are divided 

 into two parts; the lower part enables it 

 to perceive objects in the water and the 

 upper half gives it a clear view in the air. 

 The pond may include a sandy shore 

 where animals of a different kind live. 

 Snails, different from those found on the 

 mud bottom, crawl over the sand from 

 which they remove the unicellular plants 

 growing there. Frogs, toads, and turtles 

 may live around the edge of the pond, and 

 birds such as the redwinged blackbird may 

 inhabit the vegetation along the shore. Al- 

 though these animals do not live in the 

 water, they do contribute to the combined 

 interrelationships of the community. They 

 seek at least some of their food in the water 

 and when they die their bodies may fall 

 into the water where they are eaten by ani- 

 mals living there. 



It is obvious that there must exist many 

 complex food chains in such a well-defined 

 community. The chief occupation of each 

 living thing is to nourish itself, a need that 

 results in a severe struggle for existence. 

 Rarely does an animal die a natural death, 

 for the moment it wavers it is pounced 

 upon and destroyed by another, thus be- 

 coming a part of a long or short food chain. 

 There is, however, a complete food cycle 

 for the entire community which involves 

 certain general groups of plants and ani- 

 mals. The green plants always provide the 

 beginning of such a food chain. In the case 

 of the pond, the water plants extract their 

 simple needs from the water and manufac- 

 ture food which is consumed by plant feed- 

 ers, such as the tiny Crustacea that feed on 

 algae and the snails that eat larger plants. 

 These animals are pursued and eaten by 



