XI 



PHYLUM ARTHROPODA 



557 



pods 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 broaden- 

 ing 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. 



The Arthrostraca, Cumacea, and Stomatopoda may perhaps be 

 looked upon as derivatives of the Schizopod-type along distinct 

 lines of descent, the Arthrostraca showing the greatest amount of 

 specialisation, in virtue of the absence of carapace and of exopodites 

 (both present as vestiges in Anisopoda), and in the eyes being 

 sessile. The Ostracoda, Copepoda, and Cirripedia are best con- 

 ceived as derivatives, along separate lines, of an ancestral form 

 common to them and the Phyllopoda. 



These relationships are expressed in the following diagram : 



Shizopoda 



Sl - omat'o|3oda 



PhyllojDoda 

 slracoda 



Phyllocarida 



Co|ae(3oda 



Cirripedia 



FIG. 440. Diagram illustrating the mutual relationships of the orders of Crustacea. 



