vni PROTECTIVE COLOUR OF SLOTH 171 



as hooks to keep them suspended from the lower side of a 

 branch. The three -toed sloth, Bradypus (or "Ai"), has the 

 exceptional number of nine cervical vertebrae ; the two-toed sloth, 

 Choloepus lioffmanni (or " Unau "), has the equally exceptional 

 number of six. The hair is long and shaggy, and gets an 

 adventitious green colour from the presence of minute algae. 1 This 

 gives to the animal the appearance of a lichen-covered bough, a 

 resemblance which is increased in one species by an oval mark 

 upon the back, which suggests forcibly a broken end of such a 

 branch. The likeness of a Sloth to its surroundings is pointed 



Lcr 



FIG. 97. Skull of Three-toed Sloth. Bradypus tridoctylus. Lateral view, fr, Frontal ; 

 ju, jugal ; Icr, lachrymal ; may, maxilla ; nas, nasal ; par, parietal ; s.oc, snpra- 

 occipital ; ty, tympanic. (From Parker and Haswell's Zoology.) 



out by Dr. Siemann, 2 who observed that a species occurring in 

 Nicaragua " has almost exactly the same greyish-green colour as 

 Tillandsia usneoides, the so-called ' Vegetable Horsehair ' common 

 in the district. ... If it could be shown that it frequented trees 

 covered with that plant . . . there would be a curious case of 

 mimicry between the sloth's hair and the Tillandsia, and a good 

 reason why so few of these Sloths are seen." The stomach in the 

 Sloths is complicated in structure, with several chambers ; one of 

 these gives off a long crescent-shaped caecum. The skull of the 

 Sloths agrees in a number of particulars with that of the Anteaters. 



1 The colour fades in captivity owing to the disappearance of the algae. 

 - In a letter addressed to Dr. Gray, quoted by the latter in a revision of the 

 Sloths, Proc. Zool. Soc. 1871, p. 428. 



