ARTICULATED ANIMALS. 61 



of the external skeleton is visible, for in these creatures even the 

 separation between the head and the thorax becomes obliterated, 

 and it is in the abdomen only that the segments of the body are 

 recognizable. By contrasting the body of a centipede with that 

 of a scorpion, as represented in the accompanying figure (Fig. 55), 

 the progress of this coalescence of the tegumentary rings is strik- 

 ingly exemplified. 



Lastly, in the Crustacea (crabs, lobsters, &c.) we find various 

 modifications of the outward skeleton adapted to the habits of 



FIG. 56. COMMON CRAB. 



the different races. Among the lowest forms, the rings composing 

 the external framework are perfectly distinct and separate, resem- 

 bling those of the Myriapoda ; but in the stronger and more pre- 

 dacious tribes the pieces of the head and thorax become solidly 

 fixed together ; and in those forms most adapted to a terrestrial 

 life, namely, the crabs, almost all trace of distinction between the 

 thoracic segments is lost in the construction of the calcareous 

 shield which covers and protects their whole body. (Fig. 56.) 



In the animals described in preceding chapters the nervous 

 system, wherever it has been at all discernible, has existed only 

 in the form of slender threads, without being accumulated into 



