MAMMALIA. 457 



have air- vacuoles amongst them, giving rise in many cases to 

 white hair. 



The dermis surrounding the hair-germ is gradually differentiated 

 into a follicle. Its base protrudes into the hair-papilla, forming the 

 dermal papilla with blood-vessels, and the rest forms a more or less dis- 

 tinct dermic coat outside the root-sheaths. It may have an inner circular 

 layer of connective tissue and an outer longitudinal. Lastly, muscles 

 called the arrectores pili are attached to the coat and serve to erect 

 the hair. 



Hairs differ very much in structure and texture and in some aquatic 

 mammals they are almost entirely absent. 



Other epidermic structures of the same nature, i.e.., localised corni- 

 fications of the epidermis, are nails, claws, spines or bristles, horns, and 

 even scales, as in the pangolins ; horny teeth occur in the duckmole 

 and the Si7'enia. 



Hairs form a very efficient and light covering for the body and are 

 a protection mainly from climatic conditions. Indirectly, however, they 

 constitute an important protection from foes, as they are nearly always 

 of a colour in harmony with surroundings. Thus it is asserted that 

 because of their stripes the tiger and zebra in natural surroundings are 

 difficult to discern, whilst the spots of the axis deer are said to 

 exactly simulate the lights and shades formed by the sun shining 

 through leaves. The white colour of arctic animals is another example, 

 and a still more remarkable instance is that of seasonal coloration, 

 found in temperate regions. In many of the fur animals, such as the 

 polecat, weasel and ermine, the hair is of a brownish or black shade, 

 except in winter, when it becomes a pure white. In many marsupials 

 {Metatheria) the stripes are confined to the hind-quarters, as these parts 

 are most exposed when the animals are curled up asleep, and from this 

 direction an enemy can easily approach unseen. 



Mammary Glands. — The mammary glands are of uni- 

 versal occurrence throughout the Mammalia. They are 

 skin-glands, usually situated on the ventral or lower surface 

 of the animal, and their secretion (milk) is used for the 

 nourishment of the young. Whilst thus physiologically 

 distinct, they do not appear morphologically to be organs 

 sui generis. In the Monotremata the mammary glands are 

 modified from sudorific or sweat-glands, so that the 

 " milk " in these forms is sweat and is said not to differ 

 essentially in composition from this excretory product : in 

 Metatheria the mammary glands are said to be a mixture 

 of sudorific and sebaceous glands ; whilst in the higher 

 mammals {Eutheria) they consist purely of sebaceous glands 

 and the milk becomes a highly nutritive product. The 

 mammae or teats form a like series^ as there are none 



