362 ARTHROPOD A. 



poda. They resemble the Phyllopods principally in the 

 indefinite segmentation of the body, and in the likeness 

 between their hypostome and the lip-plate of the recent 

 Apus. On the other hand, they agree with the Xiphosura 

 in the possession of compound and sessile eyes, in the pos- 

 session of thoracic branchiae, and to some extent in the 



nature of their mouth - organs, so 

 far as known. Moreover, the larva 

 of the recent Limulus (fig. 218) is 

 destitute of the tail -spine of the 

 adult, and in many respects shows 

 a most striking resemblance to 

 certain of the Trilobites, and espe- 

 cially to the genus Trinudeus (fig. 

 230). There are, however, many 

 and important differences between 

 Fig. 2i8.-La of Limnius on the Trilobites and the Xipliosura. 



hatching, greatly enlarged. (After T h us ^ f ormer ^ave f ree thoracic 

 Dohrn.) 



segments, and a well-developed hy- 



postome, while there are no ocelli ; the latter, on the contrary, 

 having the thoracic segments anchylosed, the limbs entirely 

 converted into organs of prehension and mastication, the hypo- 

 stome rudimentary, and the head-shield furnished with ocelli 

 as well as compound eyes. Lastly, the Trilobites agree with 

 the Isopoda in possessing compound sessile eyes, in hav- 

 ing the abdominal somites amalgamated to form a caudal 

 shield, in the occasional power of rolling into a ball, the 

 thoracic segments being free and movable, and in the absence 

 of ocelli. They differ from the Isopods in the fact that 

 the thoracic segments of the latter are always definite and 

 almost always seven in number, and in their apparent pos- 

 session of a much larger number of locomotive appendages. 



The general facts as to the distribution of the Trilobita in 

 past time are soon told. The Trilobites are exclusively 

 Palaeozoic, their range extending from the Upper Cambrian 

 to the Lower Carboniferous. If the Palceopyge Ramsayi of 

 the Longrnynd beds be a Trilobite, then the order has its 

 commencement in the Lower Cambrian. In the Upper 

 Cambrian rocks the order attained a wonderful develop- 



