430 



PHYLUM ARTHROPODA 



wards at the anij;les ; 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. 



Traces of limbs are only rarely 

 preserved. In the head region there 

 are four pairs, apparently simple. 

 Segmented antennas have been found 

 in this region. The thorax and 

 abdomen are furnished with bira- 

 mose appendages, with long-jointed 

 endopodite, shorter exopodite. and 

 a gill (or epipodite ?) of varying 

 shape. In the abdominal region 

 the gills were perhaps rudimentar}'. 

 Trilobites are often found rolled 

 up in a way that reminds one of 

 some wood-lice. So abundant arc 

 they in some rocks that even their 

 development has been studied with 

 some success. 



The limbs seem to be more like 

 those of Crustaceans than those of 

 Arachnoids, and the 

 antennae, observed 

 (1759), and securely 

 accentuates the resemblance. The 

 affinities with Limulus, according to the views of other authorities, 

 justify the association of Trilobites and Arachnoids. A compromise 



Fig. 



242.- 

 aliies- 



--Trilobite {Conoceph- 

 — After Barrande. 



li.s., Head shield ; pL, pleura of 

 thoracic region ; py., pygidium. 



occurrence of 

 by Linnaeus 

 corroborated, 



Fig. 243. — Vertical cross-section of a Trilobite (Calymene). 



—After Walcott. 



i., Intestine ; s., shield ; L., endopodite ; e., exopodite ; b., epipodial parts. 



may be perhaps effected by regarding the Trilobites as an offshoot from 

 a stock ancestral to both Arachnoids and Crustaceans. 



