XI 



PHYLUM ARTHROPODA 



589 



From the Euphausiacea the Macrura are derivable by the 

 differentiation of three pairs of foot- jaws and the disappearance of 

 the exopodites of the legs. In the series of the Macrura we find, 

 on passing from the Prawns through such forms as Astacus, 

 Palinurus, and Scyllarus, a gradual shortening of the abdomen, 

 accompanied by a broadening and flattening of the whole body. 

 In Birgus, Hippa, &c., this process goes a step further, and the 

 abdomen becomes permanently flexed under the cephalothorax, 

 thus leading to the high degree of specialisation found in the Crabs. 



These relationships are expressed in the diagram on page 588. 



APPENDIX TO CRUSTACEA. 



Class TRILOBITA. 



The Trilobifca 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 



FIG. 484. Dalmanites socialis, dorsal aspect ; B, the same rolled up ; C, under-side of 

 head of Phacops fecundus. c.sh. cephalic shield ; e. eye ; f.c. fixed cheek ; f.s. frontal 

 suture ; gl. glabella ; Ibv. labrum ; m.c. movable cheek ; p. pygidium ; pi. pleura ; s.f.p. 

 sub-frontal plate ; th. thorax. (After Gerstaecker.) 



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. 484, c.sh), the thorax (th), and the abdomen (p), all of 

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



