COMMENSALISM AND SYMBIOSIS 177 
mussels. The mussels and the crabs live together in per- 
fect harmony and to their mutual benefit. 
There are a few extremely interesting cases of symbiosis 
in which not different kinds of animals are concerned, but 
animals and plants. It has long been known that ‘some 
sea-anemones pos- 
sess certain’ body 
cells which con- 
tain chlorophyll, 
that green  sub- 
stance character- 
istic of the green 
plants, and only 
in few cases pos- 
sessed by animals. 
When these chlo- 
rophyll -bearing 
sea-anemones were 
first found, it was 
believed that the 
chlorophyll cells 
Fie. 106.—The crab Epizoanthus paguriphilus, with 
really belonged to the sea-anemone Parapagurus pilosiramus on its 
the animal’s body, shell. 
and that this con- 
dition broke down one of the chiefest and most readily 
apparent distinctions between animals and plants. But 
it is now known that these chlorophyll-bearing cells are 
microscopic, one-celled plants, green algze, which live ha- 
bitually in the bodies of the sea-anemone. It is a case 
of true symbiosis. The algew, or plants, use as food the 
carbonic-acid gas which is given off in the respiratory 
processes of the sea-anemone, and the sea-anemone breathes 
in the oxygen given off by the alge in the process of ex- 
tracting the carbon for food from the carbonic-acid gas. 
These alge, or one-celled plants, lie regularly only in the 
innermost of the three cell layers which compose the wall 
13 
