604 ZOOLOGY sm 



These relationships are expressed in the following diagam : — 



Brachyura 



Anomur*a 

 Macrura 



Ar^hroslraca 



Anaspid 



Tri I obi ra 



OsYra 



Euphausiacea 



Shomahopoda 



Phyllocarida 



Copepoda 



Cirripedfa 



Annulaha 



Fig. 476. — Diagram illustrating the mutual relationships of the orders of Crustacea. 



APPENDIX TO CRUSTACEA. 



Class Trilobita. 



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

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

 Carboniferous. They are 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 head (Fig. 477, c.xh), the thorax (th), and the abdomen (p), all of 

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

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



