OF THE HAIK OF SLOTHS. 



339 



Probably this influence of friction is the reason why the general 

 green colour of the hair is so much better marked in some parts 

 of the animal than in others, being more especially visible at the 

 back of the head and neck. 



ID 



W 



:: .. 4^1 



fa 



mt-fe * 



rt 



gdp 1 , <•■• • 3 



11 



Fig. 1. Hair of Bradypus with algae on surface. 



2. The same, old and cracked. 



3. The same, transverse section. 



4. Hair of Cholopus, transverse section. 



5. The same, surface with algae in hollows. 



The structure of the hair of Bradypus (figs. 1, 2, & 3) differs 

 as much from that of Cholopus as from that of all other animals * 

 which have come under my observation ; and in fact we might 

 almost say that it is the reverse of the normal. Instead of there 

 being a horny sheath and a central cellular pith, there is a central 

 horny thread (fig. 3) and a pithy exterior, with somewhat oblique 

 transverse structure, made much more transparent when saturated 

 with Canada balsam. The result of this remarkable structure is 

 that old hairs break up into numerous segments (fig. 2), which 

 look like angular beads strung on a central horny thread, so as to 

 make the hair, roughly speaking, somewhat like the barbs of cer- 

 tain feathers with attached barbules. It is on this external pithy 

 portion of the hair that the green algae grow (fig. 1) ; and I am dis- 

 posed to believe that they are able to grow on it mainly because 

 the surface is sufficiently rough to allow them to attach them- 

 selves so firmly that they are not easily removed by friction. 



It will thus be seen that I attribute the presence of the algae 



[* There is a small Sloth, however, in which the larger hairs are quite 

 smooth and solid. — Ed.] 



