THE TRILOBITA AND OTHER ARTHROPODA 195 



The reason can be understood by examining the 

 methods of muscular attachment in Arthropoda. Where 

 powerful muscular action is necessary the inner face of 

 the exo-skeleton does not provide a sufficient area of 

 attachment, and to increase that area a portion of the 

 cuticle becomes, as it were, pushed in to the interior of 

 the body. Such an inward process is called an apodeme, 



rs. 



FIG. 55. CALYMENE BLUMENBACHI, BRONGNIART, WENLOCK 



LIMESTONE. 

 (Natural size.) (Original.) 



F.S., Facial suture ; Gl.%, third lateral lobe of glabella ; O.F., occipital 

 furrow; PI., pleuron. 



and in the thorax of trilobites apodemes are formed in 

 pairs at the posterior margin of each somite. The 

 lateral furrows of the glabella occur in corresponding 

 positions and are evidently apodemes of the head. Some 

 trilobites have lateral furrows much more like those of 

 the thorax than in the case of Calymene ; in others they 

 diverge still more from the primitive characters. Since 



