28 EVOLUTION 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 papilla. 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 papilla, 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 branchiz, 
intestines, foot,.nervous system, heart, reproductive 
organs, &c. 
These animals obtain their food in a somewhat lazy 
