THE TRILOBITES. 



99 



Limulus, and is often variously ornamented witli tuber- 

 cles aud spines. The body is divided into three longi- 

 tudinal lobes, the central situated over the region of the 

 heart as in Limulus. The body differs from tliat of the 



Fig. 1S4.— Younfc Horseshoe Crab. 

 Natural size and enlarged. 



Fia 135.— Young Trilobite. 

 Natural size and enlarged. 



horseshoe craL in being divided into a true head consisting 

 of six segments bearing jointed appendages, somewhat like 

 those of the Merostomata, with from two to twenty-six dis- 

 tinct thoracic segments (probably bearing short jointed 



Fig. 126.— Restored section o£ the thorax of a trilobite (Ualymene) after Wal- 

 cott. c, carapace; e)i, endopodite; fv*', exopodite, with the gills on the exo- 

 podal or respiratory part of the appendage. 



limbs not extending beyond the edge of the body). The 

 abdomen consists of several (greatest number twenty-eight) 

 coalesced segments, forming a solid portion {pygidium), 

 sometimes ending in a spina The larval trilobite (Fig. 125) 



