362 



PHYLUM ARTHROPODA. 



at the angles ; the flexible thorax of a varying number of segments ; the 

 unsegmented abdomen or pygidium. A median longitudinal ridge, or 

 rachis, divides the body into three longitudinal portions. 



.2. 



FIG. 178. Trilobite (Conocephalites\&S\ Barrande. 

 //.j., Head shield ; //., pleura of thoracic region ; jy., pygidium. 



Traces of limbs are only rarely preserved. In the head region there 

 are four pairs, apparently simple. Antenna? have been recently found 

 in this region. The thorax and abdomen are furnished with bira- 



mose appendages with 

 long -jointed endopo- 

 dite, short exopodite, 

 and a gill (or epipodite ?) 

 of varying shape. In 

 the abdominal region 

 the gills were perhaps 

 rudimentary. 



Trilobites are often 

 found rolled up in a 

 way that reminds one 

 of some wood-lice. So 

 abundant are they in 

 some rocks, that even 

 their development has 

 been studied with some 



Flo. 179. Vertical cross-section of a Trilobite 

 (Calymene). After Walcott. 



/., Intestine; s., shield ; L., endopodite ; c., exo- 

 podite ; /'. , epipodial parts. 



success. 



The limbs seem to 

 be more like those of Crustaceans than those of Arachnoids, and the 

 occurrence of antennae, observed by Linmuus (1759), and recently 



