STKUOTUKE OF THE HAIRS OF MYLODON LISTAI. 399 



the climate in which Brady pus lives enables the alga to live 

 and propagate in this curious position^ and tlie sloth acquires 

 a general green tint, which must render it very difficult to 

 distinguish as it hangs among the green foliage. In thick 

 transverse sections of the hair these algal bodies show up 

 very clearly, since they stain deeph", 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. 



In addition to the larger hairs just described, Bradypus 

 has a set of shorter and much finer hairs, constituting the 

 under-fur. These hairs have a diameter oF 24//, and consist 

 of a column of cortical substance traversed by fine fusiform 

 air spaces, and covered by an imbricated cuticle (fig. 7). 

 Like the larger hairs of the body, they have no medulla. 



Choloepus didactylus. 



The hairs of Cholcepus are no less remarkable than those 

 of Bradypus, but in a totally different way. The bulk of 

 the hair is composed of cortex, the surface of which is fluted 

 or channelled. The grooves, as is well known, are occupied by 

 strands of extra-cortex, in which lives an alga — the Pleuro- 

 coccus Cholcepi of Kiihn (17, p. 6Q). Even from the hairs 

 of dried museum specimens a green solution, giving the 

 absorption bands of chlorophyll, can be obtained by boiling- 

 first in water and then in alcohol. 



When seen in transverse section (fig. 8) the outline is oval, 

 and measures about 150// x 90/t. The cortical substance is 

 in some cases quite clear and hyaline, but in others it is 

 marked by brown spots — differences presumably related to 

 the age of the hairs. In both cases this cortical substance 

 does not stain with magenta. But running throughout, 

 except towards the summits of the superficial ridges of the 

 hair, are irregular branching lines, which stain deeply, and 

 are discernible in unstained sections by reason of their 

 different refrangibility. In very thin sections these lines are 



