MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 377 



the sea anemone breathes in the oxygen given off by the algae 

 in the process of extracting the carbon for food from the car- 

 bon dioxide. These alga?, or one-celled plants, lie regularly 

 only in the innermost of the three cell layers which compose 

 the wall or body of the sea anemone (Fig. 231). They penetrate 

 into and lie in the interior of the cells of this layer, whose special 

 function is that of digestion. They give this innermost layer 

 of cells a distinct green color. Even certain amoebalike 

 protozoans have been found to contain individuals of a one- 

 celled alga, Chlorella, 

 in their single-celled 

 bodies, the tiny ani- 

 mal and smaller plants 

 living together truly 

 symbiotically. 



Among the higher 

 plants and animals, 

 cases of symbiosis are 

 not rare. There lives 

 in the live-oak trees 

 in the vicinity of Stan- 

 ford University a cer- 

 tain scale insect, Cero- 

 coccus ehrhorni, which 

 differs from the other 

 two or three species of 



its genus in not having its body covered by a heavy, thick, 

 protecting layer of secreted wax. But it gets the needed 

 protection in another way. It is always covered by a thick 

 felt like fungus growth, which has been found by investiga- 

 tion to germinate its spores and to find a constant food 

 supply in the "honey dew' excreted by the scale insects. 

 This feltlike covering of fungus, never found to be lacking 

 in the scale insect, serves apparently as a sufficient sub- 

 stitute for the heavy waxen mass common to the related 

 species. 



The ants show particularly well instances of interesting 

 symbiotic life with plants. Fig. 232, drawn from a specimen 

 sent to us from the Philippine Islands by the botanist Cope- 

 land, shows some details of one such instance. The Dis- 

 chidias are milkweeds of the extreme Orient. They twine 



FIG. 231. Diagrammatic section of sea anemone: 

 a, The inner cell layer contains alga cells, the 

 two isolated cells at the right being cells of this 

 layer with contained algse; 6, middle body wall 

 layer; c, outer body wall layer. (After Hertwig.) 



