APRIL 59 



Of course protective coloration is not a monopoly of 

 feathered animals. The purpose is evident in the 

 adaptation of many mammals, reptiles, fishes, and 

 insects to their surroundings, but perhaps the most 

 curious instance among vertebrate animals is the case 

 of the three- toed sloth (Brady pus) as described by Dr. 

 W. G. Ridewood (Quarterly Journal of Microscopical 

 Science, vol. xliv.). The sloth in question is not a 

 microscopical object ; far from it ; but the hair of the 

 creature shows a very complex structure under the 

 microscope. Each hair consists of a core, a cortex or 

 skin, and an outer coat. This outer coat splits, and in 

 the fissures grows a green alga peculiar to this situa- 

 tion, and therefore known to science as Pleurococcus 

 bradypi. This little alga (something between a fungus, 

 a moss, and a lichen) grows so fast in the moist heat of 

 the Brazilian forest as to impart a green coloration to 

 the whole of this large animal, which derives distinct 

 advantage from such an assimilation in hue to its sylvan 

 environment. 



The two-toed sloth (Cholcepus) enjoys a similar 

 privilege, but this is provided by a vegetable growth 

 different from that on the three-toed sloth. The hair 

 of Chol&pus consists only of core and cortex, without 

 the extra layer grown by Bradypus. This cortex, or 

 rind, is grooved or fluted longitudinally, and in these 

 grooves springs a growth of another species of alga 

 Pleurococcus cholospi. Drawings of the hairs of these 

 sloths, enlarged, are shown in the sloth case in the 

 Natural History Museum, South Kensington. 



