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 pild are attached to the coat and serve to erect 
the hair. 
Hairs differ very much in structure and texture and in some aquatic 
mammials they are almost entirely absent. 
Other epidermic structures of the same nature, z.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 Szrenza. 
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 » 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 GLAnps.—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 (Zutheria) they consist purely of sebaceous glands 
and the milk becomes a highly nutritive product. The 
mammz or teats form a like series, as there are none 
