EPIDERMAL STRUCTURES. 



offence or defence ; or again may be of value at the rutting 

 season, as attractions for the other sex. Of the defensive 

 glands those of the skunk and polecat come immediately to 

 mind ; to the other category belong the peculiar glands of the 

 beaver, civet cat, musk deer, etc. These glands may be placed 

 near the eyes (deer), on the back (musk swine), on the legs 

 (ordinary swine), on the ventral surface (musk deer), or near 

 the vent (skunks, etc.). 



A more extreme modification of 'the sel^aceous glands is 

 found in the milk glands of all mammals except monotremes, 1 

 the secretion of which 

 serves to nourish the 

 young. These milk 

 glands are in pairs 

 upon the ventral sur- 

 face, the number being 

 roughly correlated to 

 the number of young 

 brought forth at a time. 

 The ducts of each 

 group of glands open 

 upon a limited extent of surface, and this becomes converted 

 into a teat or nipple, either by the elevation of the skin in 

 which the ducts open (Fig. 97, A}, or by the extension of the 

 surrounding skin into a tubular form (B). 



In the skin, pigment is of common occurrence. It may be 

 found in the epidermis, but is more common in the derma. It 

 may consist of scattered pigment granules, or there may be 

 special pigment cells (chromatophores) which enlarge or con- 

 tract under control of the nervous system, producing those 

 color changes so noticeable in many lizards, and to a less degree 

 in amphibia and fishes. 



EXOSKELETON. 



Either or both layers of the skin may participate in the 

 formation of firmer parts constituting a tegumentary skeleton, 

 which may take the form of scales or bony plates ; and it is to 



1 The milk glands of the monotremes are apparently derivatives of sweat glands. 



FIG. 97. Two types of nipple. 



