378 



PHYLUM ARTHROPODA. 



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



are four pairs, apparently simple. 

 Antennae 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 

 antennae, observed by Linnaeus 

 ( 1 759)> an( * recently corroborated, 

 accentuates the resemblance. The 

 affinities with Lifnulus, according 

 to the views of other authorities, 

 justify the association of Trilobites 

 and Arachnoids. A compromise 

 may be perhaps effected by regard- 



FIG. 201. Trilobite (Conoceph- 

 alites). After Barrande. 



k.s. t Head shield ; /*/., pleura of 

 thoracic region; py. t pygidium. 



ing the Trilobites as an offshoot from a stock ancestral to both 

 Arachnoids and Crustaceans. 



FIG. 202. Vertical cross-section of a Trilobite (Calymene). 



After Walcott. 

 *., Intestine; j., shield ; Z-., endopodite; e., exopodite; ., epipodial parts. 



Incerta Sedis 

 Class PYCNOGONIDA, PANTOPODA, or PODOSOMATA 



Marine Arthropods, sometimes called sea-spiders. They may be 

 ranked between Crustaceans and Arachnoids. Many climb about 



