a PHYLUM ARTHROPODA 003 



separate lines, of an ancestral form common to them and the 

 Branchiopoda. By a differentiation of the post-cephalic limbs, 

 ami a reduction in the number of segments, the branchiopod- 

 type easily passes into that of the Phyllocarida, which, though 

 they nearly conform to the malacostracan type of segmen- 

 tation, have still marked traces of relationship with lower 

 groups in the presence of caudal styles and in their bivalved 

 carapace and foliaceous thoracic appendages. Next to these in 

 ascending order would come the Cumacea with their cephalic 

 carapace coalescent with the first three or four thoracic 

 segments and bounding branchial cavities at the sides of the 

 thorax, but with — as more primitive features — a biramous character 

 in some of the thoracic appendages and the absence of the fan- 

 like tail-fin. Then a little higher, the Arthrostraca (Tanaidacea, 

 Isopoda and Amphipoda) and the Anaspidacea may be supposed 

 to have branched off from the main trunk at about the same level, 

 and may be regarded, on account of a number of resemblances, as 

 having had a common origin from it. Probably the Anaspidacea 

 are to be looked upon as more primitive than the other two groups 

 in view of their less advanced coalescence of the first thoracic 

 segment with the head, the absence of specialised maxillipedes, 

 and the biramous character of the thoracic limbs ; but, on the 

 other hand, they show a higher development in the possession of 

 the fan-like tail-ifin and the stalked movable eyes such as charac- 

 terise the Decapoda. 



A stage nearer the latter group are the Mysidacea, with their 

 single pair of maxillipedes, their stalked eyes, their rudimentary 

 podobranchia? and their fan-like tail-fin ; but these still show some 

 primitive features, more especially in their incomplete cephalo- 

 thorax and their biramous thoracic appendages. But without 

 doubt it is in the Euphausiacea that we find the nearest connections 

 with the Decapoda. This is shown, in spite of the absence of 

 maxillipedes, in their completed cephalothorax, their series of 

 podobranchiaB, and sac-like heart, in addition to their stalked eyes 

 and fan-like tail-fin. 



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. 



