METAZOA ARACHNOZOA. 363 



890, dorsal side). The body is also divided longitudinally 

 into three lobes central axis with pleurae on either side 

 and hence the name of trilobite. 



The cephalon is provided with compound unstalked 

 eyes. These eyes have migrated, as we have already 

 said, from the ventral side over the margin and are now 

 on the inner side of the free cheeks some distance from 

 the margin. Each segment bears a pair of appendages, 

 most of which are similar in structure and adapted for 

 swimming or for crawling on the sea bottom. 



In front a pair of long, jointed antennae have been dis- 

 covered. These are clearly seen extending forward in 

 No. 889, a specimen taken from the lower Silurian forma- 

 tion. 



A deep groove runs through the middle of the ventral 

 side of Triarthrus becki (No. 891), and the mouth parts 

 and long, jointed feet fastened to the axis conceal the tri- 

 lobed character of the body. 'These feet are made up of 

 a stem and two branches, one of which is adapted for 

 swimming, having long hairs, while the other is fitted for 

 crawling. The appendages of the pygidium are especially 

 fitted for locomotion, having flattened leaf-like sections 

 and very long hairs. 



According to Beeches 1 no traces of any special respir- 

 atory organs have been found in Triarthrus and their ex- 

 istence is doubtful, though the fringes on the locomotor 

 organs may have served as gills, since in many forms the 

 functions of locomotion and respiration were combined. 



In some genera of trilobites the central axis is broad, 

 while the cephalon is small and granulated, as seen in 

 Lichas boltoni Hall (No. 892). Many of the hairy ap- 

 pendages are well preserved in this fossil, and are seen 

 lying on either side of the large flat body. The glabella 

 has a swollen lobe in front besides lateral lobes, and the 

 eyes are seen near the margin. 



1 Amer. Journ. Sci., (4), I, April, 1896, p. 253. 



