178 



ANIMAL LIFE 



or body of the sea-anemone (Fig. 107). They penetrate 

 into and lie in the interior of the cells 'of this layer whose 

 special function is that of digestion. They give this inner- 

 most layer of cells 

 a distinct green 

 color. 



There are other 

 examples known of 

 the symbiotic asso- 

 ciation of plants 

 and animals ; and 

 if we were to fol- 

 low the study of 

 symbiosis into the 

 plant kingdom we 

 should find that in 

 one of the large 

 groups of plants, 

 the familiar lichens 

 which grow on 



rocks and tree trunks and old fences, every member lives 

 symbiotically. A lichen is not a single plant, but is always 

 composed of two plants, an alga (chlorophyll-bearing) and 

 a fungus (without chlorophyll) living together in a most 

 intimate, mutually advantageous association. 



FIG. 107. Diagrammatic section of sea-anemone, a, 

 the inner cell layer containing alga cells, the two 

 isolated cells at right being cells of this layer with 

 contained algae ; b, middle body wall layer; c, outer 

 body wall layer. After HERTWIG. 



