THE TR1L0B1TE8, 
99 
Limtdits, and is often variously ornamented with tuber- 
cles and sjoines. The body is divided into three longi- 
tudinal lobes, the central situated over the region of the 
heart as in Limulus. The body difiEers from that of the 
Fig. 124.— Youn.G: Horseshoe Crab. Fig. 135.— Young Trilobite. 
Natural size and enlarged. Natural size and enlarged. 
horseshoe craF^ in being divided into a true head consisting 
of six segments bearing jointed appendages, somewhat like 
those of the Merostoniata, with from two to twenty-six disr 
tinct thoracic segments (probably bearing short jointed 
Fig. 126.— Restored section of the thorax of a trilobite (Calymene) af ter Wal- 
cott. c, carapace; en, endopodite; en', 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 spine. The larval trilobite (Fig. 125) 
