588 



ZOOLOGY 



SECT. 



primitive than the other two groups in view of the 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 develop- 

 ment in the possession of the fan-like tail-fin and the stalked 

 movable eyes such as characterise the Decapoda. 



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

 single pair of maxillipedes, their stalked eyes, their rudimentary 



Brachyura 



Macrura 



Arl"hroshraca 



Mysidacea 

 Anaspidacea 



Trilobiha 



Branchiopoda 



Osfracoda 



Anomu'r'a 



Euphausiacca 



St~omal~opoda 



Phyllocarida 



Copepoda 



Cirripedi'a 



FIG. 483. Diagram illustrating the mutual relationships of the orders of Crustacea. 



podobranchise 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 

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

 and fan-like tail-fin. 



