THE AGE OF INVERTEBRATES OF THE SEA. Si 



is dominant. Seen from above, it presents three divisions from 

 front to rear : — first, a cephalic shield or head-piece ; secondly* 

 a thorax, divided into several segments movable upon each 

 other ; and thirdly, a tail-piece or pygidium, which, when brought 

 against the head by the rolling up of the body segments, effec- 

 tually covers the lower parts. This lower portion was until 



Fig. 76rt.— Burrows of Trilobite and of modern King crab. The Trilobite burrow is known 



as Ruschimtes. 



lately little known ; but the discoveries of Billings and of 

 Wolcott have enabled us to restore the jaws under the head, 

 the jointed legs and spiral gills under the thorax, and thus to 

 complete the structure of the animal, and understand better its 

 relations to modern crabs and shrimps (Fig. 76). Of these it 

 certainly comes nearest to the King-crabs and Horseshoe-crabs, 



