THE CRUSTACEA 



and often also in the same animal. They may be nearly all alike 

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

 pairs may be specialised as sensory or as grasping organs. Com- 

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

 modified to assist the oral appendages and are known as maxillipeds. 

 It is very characteristic of the Malacostraca that the series of 

 trunk- limbs is differentiated into two well-defined "tagmata" or 

 groups of similarly modified appendages, corresponding to the 



B 



FIG. 9. 



A, maxillula of Copepod (Calanus). (After Sars.) B, maxillula of Crayfish : C, maxilla of 

 Crayfish. (After Huxley.) en, endopodite ; ep.'epipodite ; ex, exopodite ; gn, gnathobasic lobes. 

 (The plate lettered cp in C is possibly the exopodite rather than the epipodite ; see p. 2G8.) 



thoracic and abdominal regions respectively. The thoracic limbs 

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

 walking-legs, and the exopodites, 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 subsidiary groupings. Thus, for example, in 

 the Decapoda a group of three pairs of maxillipeds is differentiated 

 from the thoracic tagma. 



