MESSMATES, MUTUALISTS, AND PARASITES 75 
eft, upon; phyton, a plant). The advantage to the Orchids is 
obvious, though they do not absorb the sap of the plants upon 
which they live. These last, however, apparently derive no 
benefit from this one-sided arrangement. 
There are numerous cases of commen- 
salism between plants and animals in which 
the latter alone are benefited. In one of the 
Liverworts (/rullania dilatata, fig. 1069) 
which grow on tree-trunks there are little 
cup-like outgrowths on the under sides of 
the leaves, serving as the abodes of a species 
of Wheel-Animalcule (Ca/idina symbiotica). 
Marine plants often bear animals as mess- 
mates, which do them no harm. Ona kind 
of brown sea-weed (Fucus serratus), for ex- 
ample, are frequently to be seen the little 
spiral tubes of a sort of Annelid (SAzvo0rb7s), 
which no doubt secures an increased supply 
of nutriment and dissolved oxygen by being 
moved about in the water when the tide is 
up. The Australian Sea-Horse (Phy/lop- 
teryx egues) also benefits by its association 
with the sea-weeds to which it bears a 
resemblance (see vol. ii, p. 296). 
Some instances are known of commen- 
salism between a plant and an animal, in 
which both derive advantage from the as- 
sociation. Ant-plants illustrate such an 
arrangement (see p. 81), and so do the 
Sloths of South America, in which minute . 
alge live in the grooves of the fluted eo ee 
hairs, For these aleve are provided with oe ee ne nail 
a sheltered home, and at the same time — dravins is of one cup withits Router 
give a greenish tint to the hairs, the 
Sloths being thereby rendered less conspicuous to their enemies. 
Mutuatism (Symbiosis).—Organisms living together as Mutu- 
alists are very intimately associated for mutual benefit. Mutualism 
between two plants is well illustrated by leguminous forms and 
the minute fungi which live in the tubercles on their roots. And 
every Lichen may be regarded as a joint-stock community, con- 
