GENERAL LAWS OF ANIMAL STRUCTURE. 267 



Of these two positions of the skeleton, which is best? 

 For motion and locomotion they are probably equally 

 good ; but the external skeleton has the great advantage 

 of being protective as well as locomotive, while the in- 

 ternal skeleton has the much greater advantage of leav- 

 ing the surface sensitive to external impressions, and 

 therefore an inlet to knowledge of external nature, 

 and thus gives greater capacity for higher development. 



So much for the general plan. Now the details of 

 the homology. 



Serial Homology. — In the case of vertebrates we 

 took up first special homology, because this is by far the 



Fig. 169.— Diagram section across an arthropod, showing the inclosing 

 skeleton ring and a pair of jointed appendages : «, nervous center ; 

 v, viscera ; 6, blood system. 



most distinct. In the case of Articulata we take up only 

 serial homology, because not only is this the most dis- 

 tinct, but it includes the other also ; for we may regard 

 each articulate animal as consisting of a series of seg- 

 ments essentially similar, but modified according to its 

 place and function in the series, and then the structure 

 thus formed is again modified according to the place in 



