ARTICULATION. 347 



members, metamera, or trunk-segments. In the same way 

 in which the articulate and the externally articulated 

 Worms developed from an inarticulate condition, so the 

 internally articulated Vertebrate proceeded from an 

 originally inarticulate condition. We shall presently ex- 

 amine more closely the living representative of this con- 

 dition, the Ascidia, a remarkable class of inarticulate Worm 

 forms. (Chapters XIII. and XIV.) 



This process of articulation or metameric formation is, I 

 repeat, of the highest importance in enabling us to under- 

 stand each higher animal form, not only in its morpho- 

 logical, but. also in its physiological relations. This articu- 

 lation is one of the most important conditions necessary 

 to perfection : it is one of the principal causes of the 

 complex body-functions of higher animals. The inarticulate 

 animal can never attain so high a degree of perfection in 

 form or in function as the articulated. And the reason is- 

 plain. These members, or metamera, are, in a certain sense, 

 independent individuals. By division of labour, these- 

 originally homogeneous individuals develop into the different 

 parts of the composite body-person, just as the embryonic 

 cells fashion themselves, in consequence of division of labour, 

 into the various tissues. The body of articulated animals 

 may be likened to a railway train, in which the individual 

 carriages, held together by the couplings, represent the- 

 metamera. The engine is the head of this articulated 

 organism. Then come tender, mail-van, luggage-vans, 

 passenger-carriages, cattle-tracks, etc. Each separate 

 waggon or carriage is morphologically an individual, and 

 physiologically, yet the entire chain presents only a single 

 individual, the railway train. As in this instance the 



