96 



THE RISE OF ANIMAL LIFE 



&••.»••»»►•/"• I , 



Fig. 5-8. Animals become associated rather intimately in many >vays. Here are some illustrations of some of these 



associations. 



among the tentacles of the jellyfish which 

 offer protection by their stinging cells. The 

 benefit to the jellyfish comes from food 

 brought by the fish during its sojourns in 

 the surrounding waters. In all cases of com- 

 mensalism the relationship is a loose one, 

 and neither party is forced to live in co- 

 operation with the other. 



Mutualism. A situation where animals 

 live tofjether with benefit to both is called 

 mutualism. There are all gradations of this 

 association, from those who only casually 

 meet and become associated to those that 

 are always found together and, indeed, can- 

 not live apart. One illustration of the tem- 

 porary associations is that of hermit crabs 

 and sea anemones (Fig. 5-8). In this case 

 the sea anemone, attaching itself to the 

 shell occupied by the crab, gets free trans- 

 portation to areas which the crab finds at- 

 tractive because of an abundance of food. 

 In return for the ride the sea anemone acts 

 as a camouflage, making the shell resemble 

 the rest of the ocean floor. In addition, be- 

 cause of its powerful battery of stinging 



cells, it functions as a line of defense 

 against possible enemies of the crab. Some 

 primitive chordates ( prochordates ) have 

 adopted the same association with hermit 

 crabs (Fig. 5-8). 



Another interesting association, where 

 the relationship is more or less compulsory, 

 is the case of the metazoan, hydra, and 

 a unicellular plant, alga (Fig. 5-9). The 

 algae live in the hydra's inner layer of 

 cells (endoderm) where they carry on 

 photosynthesis, releasing oxygen which is 

 utilized by the hydra. The hydra in turn 

 releases carbon dioxide which is used by 

 the algae. While in nature this situation us- 

 ually exists, it has been possible to separate 

 them in the laboratory and each can sur- 

 vive without the other. 



In some cases of muti^ialism the associa- 

 tion of two animals is so intimate that 

 neither can live without the other. The best 

 illustration of this is among the termites, 

 or white ants (Fig. 5-9). This association 

 came to the attention of biologists when 

 it was discovered that termites could sur- 



