300 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



example, four apodemes are well developed between 

 every two thoracic somites \ the inner pair unite above 



and below so as 

 to form a closed 

 canal, the ster- 

 nal canal ; with 

 these, on either 

 side, one of the 

 outer apodemes 

 becomes con- 

 nected, and, as 

 it also becomes 

 connected with 

 the apodemes 

 behind it, the 

 several parts are 

 united into a 

 continuous and 

 substantial in- 

 ternal support- 

 ing and protec- 

 tive mass, which, 

 in addition to its 

 other functions, 

 affords attach- 

 ment to mus- 

 cles. 



The skeleton 

 may gain in pro- 

 tective or defen- 

 sive power by 

 the development 

 of spines, pro- 

 Fig. 124. The King-Crab (Limulus moluccanus). cesses, or knobs, 



which resist the 



attacks of enemies, or are able to passively inflict in- 

 jury upon them ; sometimes, indeed, they almost come 



