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- 
Fie. 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 alge; 5, middle body wall layer; c, outer 
body wall layer.—After Hentwia. 
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. 
