THE CRUSTACEA 



13 



aiul often also in the same animal. Tliey may be nearly all alike 

 as in the Branchiopoda, where, at most, one or two of the anterior 

 j)airs may be specialised as sensory or as grasping organs. Com- 

 monly, as in the Copepoda, one or two of the anterior pairs are 

 nioditied to assist the oral appendages and are known as maxiUi'pcds. 

 It is very characteristic of the Malacostraca that the series of 

 trunk-limbs is ditt'erentiated into two well-defined " tagmata " or 

 groups of similarly modified appendages, corresponding to the 



qn. 



Fio. 0. 



A, iiiaxillulii of C'o))epod (C'n/«)iHs). (After Sars.) li, maxillula of Crayfish ; C, maxilla of 

 Craylish. (After Huxley.) en, endopodite ; cj), ei)ipodite ; tx, exopodite ; ijn, giiathobasie lobes. 

 (The plate lettered f;» in C is possibly the exopodite rather than the epipodite ; see p. -JOS.) 



thoracic and alxlominal regions respective]^^ The thoracic limbs 

 have the endopoditcs forming, as a rule, more or less efficient 

 walking-legs, and the e.xopodites, when present, form swimming: 

 branches, while the abdominal limbs are usually biramous, with the 

 rami similar and, in the more primitive types, natatory in function. 

 The general similarity between the appendages of each tagma is 

 usually qualified by minor modifications, sometimes leading to 

 the formation of sul>sidiary groupings. Thus, for example, in 

 the Decajwda a group of three pairs of maxillipeds is differentiated 

 from the thoracic tajrma. 



