VERTEBRATES 3 



its points of contact with the ground the greater will be its effectiveness. 

 In the Primates the digits may be used for prehensile purposes; in the 

 flying vertebrates, the birds and bats, the fore limbs become wings. 



The tail of vertebrates is the postanal portion of the body, and is 

 the principal organ of locomotion in most aquatic forms. In the 

 terrestrial representatives it has lost its primitive significance and 

 varies much among the various groups in relative length as well as in 

 function, and may be rudimentary or absent altogether. 



The integument of vertebrates, which forms the outer covering 

 of the body, consists of two distinct layers, the outer epidermis and 

 the inner dermis, and its effectiveness as a protective envelope is 

 increased by the growth of additional characteristic coverings, such as 

 the bony dermal scales of fishes and the horny epidermal scales and 

 plates of reptiles, feathers of birds and hair of mammals. The epi- 

 dermal layer of the integument is soft and slimy in fish and amphibians, 

 which live in or near the water, while in terrestrial vertebrates, whose 

 bodies are exposed to the air, it becomes more or less hard and horny 

 and the special integumental coverings of the land forms just men- 

 tioned appear as specializations of it. 



One of the most conspicuous of the external characters of verte- 

 brates, as well as of other animals, and consequently important in 

 the classification of them, is their coloration. This is also a feature 

 of great importance to the animal itself in its relation to its environ- 

 ment, inasmuch as in a large class of cases the coloration of an animal 

 tends to render it inconspicuous to its enemies and to its prey, and 

 perhaps to increase the absorption of heat from the atmosphere or to 

 prevent the too great radiation of bodily heat to it. The same color 

 pattern, however, in numerous cases does not characterize all the indi- 

 viduals of a species, the males being often colored differently from the 

 females and the young individuals from the adults. The summer coat is 

 also often different from the winter coat in color and markings. The 

 practical purposes of these differences is sometimes hard to explain, but 

 can often be correlated with the habits of animals and the need of con- 

 cealment and protection at different times of the year and during dif- 

 ferent periods of growth. A few species are dichromatic, there being 

 two color phases which may characterize individuals. The gray 

 squirrel and the black bear are familiar examples, gray and black 

 squirrels appearing in the same litter and black and brown (cinnamon) 

 individuals in the same family of bears. ^Metachrosis, or the rapid 

 change in color under the influence of external stimuli, is common 

 among fishes, amphibians and lizards. Brown and green tones may 



