604 ZOOLOGY 



These relationships are expressed in the following diagam :- 



Brachyura 



Anomufa 

 Macrura 



A^^h^os^^aca 



Anaspid 



Tn'lobiha 



Osh 



Euphausiacea 



Sl"oma^opoda 



Phyllocarida 



Copepoda 



CIrripedia 



Annulaha 

 Fig. 476. — Diagram illusti-ating the mutual relationships of the orders of Crustacea. 



APPENDIX TO CRUSTACEA. 



Class Trilobita. 



The Trilobita are extinct Arthropods peculiar to, and characteristic of, the 

 PaliEozoic rocks : they are specially abundant from the upper Cambrian to the 

 Carljoniferous. They aie often found in a wonderfully good state of preservation, 

 owing to the hard exoskeleton covering the dorsal surface : the greater part of 

 the ventral region and the appendages were, however, very delicate, and are 

 preserved only in exceptionally favourable cases. 



The body is depressed, more or less oval in outline, and divided into three 

 regions, the hear/ (Fig. 477, c.xh), the thorax (th), and the abdomen ()>), all of 

 uiiich usually present an elevated median ridge and depressed lateral portions, 

 wheuce the trilobation generally characteristic of the group. The head is 



