28 E VOLUTION AND DISEASE. 



when not kept clean ; grubbing about in dirt and 

 muddy, stagnant pools incidental to farmyards will 

 produce warts on the noses of lambs. 



The hairless pads on the feet of carnivorous mammals 

 are made up of closely packed enlarged papillae. When 

 confined in cages and not kept scrupulously clean, the 

 combined effects of dirt and limited use often induces 

 a growth of warts. Sometimes, especially in the coati- 

 mundi, the whole of the pad will be covered with 

 elongated papillae, the appearance of such feet reminding 

 us of the pad on the plantar aspect of the ostrich's toes. 



The way in which skin responds to external stimuli 

 explains the manifold modifications it presents in the 

 various classes of animals, and it is highly probable that 

 dermal structures of great utility to individual animals 

 have arisen under circumstances such as produce them 

 in man under abnormal conditions. 



This response of the skin to irritation or abnormal 

 stimulus is not confined to vertebrates ; the lamelli- 

 branchs illustrate it in a striking way. 



Lining the concavity of the shells is a membranous 

 structure, which may be regarded as the integument, 

 and is known as the pallium or mantle. The shell itself 

 is the direct result of the excretory efforts of the lobes 

 of the mantle, and is composed of animal matter 

 hardened by deposits of carbonate of lime. 



Occupying the space between the mantle of opposite 

 sides, we find the animal proper, consisting of branchiae, 

 intestines, foot, nervous system, heart, reproductive 

 organs, &c. 



These animals obtain their food in a somewhat lazy 



