ii6 



CHORDATE ANATOMY 



terns, therefore, serve to convict men of animal ancestry, as they have on 

 occasion served to convict them of crime. 



On the sides of the fingers, the friction-ridges merge into rows of wart- 

 like elevations. This has been interpreted as confirming the opinion that 

 the ridges are remnants of rows of horny scales. That the ancestors 



^ FRICTION 

 ^DIGITAL PADS^ _^^RIDGES 



m 



TRIRADIl 



TRIRADII 



MNTERDIGITALjl, 

 PADS 



^1- 



Y 



THENAR.< 

 PAD \ 



.WPOTHENAR N, 



PAD HYPOTHENAR ^ 



PAD 



A. INSECTIVORE B. MONKEY C. MAN 



Fig. io8. — Friction-ridge patterns in three mammals — insectivore, monkey, and 

 man. The presence of such useless and rudimentary concentrically arranged ridges in 

 the human hand receives its only reasonable interpretation in the light of the evolution 

 theory. (Redrawn after Wilder.) 



of the mammals were scaly is, however, supported by more convincing 

 evidence than this. 



APPENDAGES OF THE INTEGUMENT 



Throughout almost the entire animal kingdom the skin tissues form 

 various calcareous, chitinous, or horny structures — shells, spines, teeth. 



SPINE OENTI IE 



EriAriEL ORCAN 



COMPACT 



CONNECTIVE 



TISSUE 



LOOSE 



'connective 



TISSUE 



BASAL PLATE 



B. D. 



Fig. 109. — A vertical section of the skin of an elasmobranch, showing five stages in 

 the development of a placoid scale. The development of a placoid scale is essentially 

 like that of a tooth. This fact taken together with the similarity of their structure sug- 

 gests that teeth may have evolved from placoid scales. (Redrawn after Schimkewitsch, 

 modified.) 



bones, scales, hair, feathers, horns — which serve for defense, support of 

 tissues, or attachment of muscles. The limy shells of molluscs and the 

 chitinous exoskeletons of arthropods serve all three purposes. 



Among vertebrates, the placoid scales, which first appear in certain 

 sharks of the Upper Devonian, are especially important because of their 



