378 PHYLUM ARTHROPODA. 
Traces of limbs are only rarely preserved. In the head region there 
Fic. 201.—Trilobite (Conoceph- 
alttes).— After Barrande. 
h.s., Head shield ; 4/., pleura of 
thoracic region; Ay., pygidium. 
are four pairs, apparently simple. 
Antennz have been recently found 
in this region. The thorax and 
abdomen are furnished with biram- 
ose appendages, with long-jointed 
endopodite, 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 success. 
The limbs seem to be more like 
those of Crustaceans than those of 
Arachnoids, and the occurrence of 
antenne, observed by Linnzus 
(1759), and recently corroborated, 
accentuates the reseniblance. The 
affinities with Zzmzlus, according 
to the views of other authorities, 
justify the association of Trilobites 
and Arachnoids. A compromise 
may be perhaps effected by regard- 
ing the Trilobites as an offshoot from a stock ancestral to both 
Arachnoids and Crustaceans. 
Fic. 202,—Vertical cross-section of a Trilobite (Calymene). 
—After Walcott. 
z., Intestine; s., shield ; Z., endopodite; ¢., exopodite; 4., epipodial parts. 
Incerte Sedis 
Class PYCNOGONIDA, PANTOPODA, or PODOSOMATA 
Marine Arthropods, sometimes called sea-spiders. They may be 
ranked between Crustaceans and Arachnoids.. Many climb about 
