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 a'i, 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 a'i, 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 



