SLOTHS AND THEIR HAIR 317 
one hand to the funguses and on the other to the lichens. 
The majority live in water—either salt or fresh—compara- 
tively few deriving their nourishment from the moisture 
contained in the air. Some, indeed, are confined to particular 
descriptions of rock, and possess structures recalling roots, 
but even in these cases it is doubtful if they draw more 
than an insignificant fraction of their nutriment from the 
substance on which they grow. 
In the moist tropical forests forming the home of the 
sloths the algas in the cracks of their hairs grow readily, 
and thus communicate to the entire coat that general green 
tint which, as already said, is reported to render them 
almost indistinguishable from the clusters of lichen among 
which they hang suspended. 
“In thick transverse sections of the hair,” writes Dr. 
Ridewood, who has recently investigated the structure of 
sloth-hair, ‘‘these algal bodies show up very clearly, since 
they stain deeply, and have a sharply defined circular or 
slightly oval outline. Unless the hair is much broken, they 
are confined to the outer parts of the extra-cortical layer.” 
Not the least curious phase of a marvellous subject is 
that the two-toed sloth, although the structure of its hair 
is very different from that of the ai, also has an alga, 
which belongs to a species quite distinct from the one 
found in the former. 
In the two-toed sloth the hairs lack the outer sheath 
investing those of the ai, and consist chiefly of the central 
core or cortex; in other words, they correspond to those 
hairs of the latter from which the outer sheath has been 
shed. The surface of these hairs is distinctly furrowed 
with longitudinal grooves or channels, and it is in these 
channels that the alga distinctive of this particular species 
is lodged and flourishes. After stating that a solution 
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