MUTUAL AIT) AXD COMMUNAL 1.11 i: AMONG AXLMALS 



4 t 



the sea anemone breathes \n tlie oxygen given ofY \)y tlie algio 

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

 bon dioxide. These alga*, or one-celled j)lanls, lie regularly 

 only in the innermost of tlie tliree cell layers whicli compose 

 the wall or body of the sea anemone (Fig. 2'M). 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 anKebalikc; 

 protozoans have been found to contain individuals of a one- 

 celled alga, Chlorclla, 

 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 b}' a heavy, thick, 

 protecting layer of secreted wax. Hut it gets the needed 

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

 feltlike fungus growth, which lias ])een 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 felt like covering of fungus, never found to be lacking 

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

 stitute for the heavy, waxen mass conunon to the related 

 species. 



The ants show ])articularly well instances of interesting 

 symbiotic life with plants. Fig. 2o2. 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. Tliev twine 



Fig. 231. — Diagrammatic section of sea anemone: 

 a. The inner cell layer contain.s alga cclLs, the 

 two isolated eelLs at the right heirig cells of thi.s 

 layer with contained alg:r; 6, midiiie b<jdy wall 

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



