44 THE APODID^: PART 



THE LIMBS OF THE TRUNK. 



Passing from the second maxillae to the first trunk 

 limb, we are struck by a sudden contrast, the former 

 being much reduced, while the latter is highly de- 

 veloped, indeed the most highly developed limb of 

 the whole body. This point is of more than ordinary 

 interest, as we shall find that it throws considerable 

 light on the homologies of the limbs in the Xipho- 

 suridse, the Eurypteridas, and the Trilobites, in which 

 animals, from what we learn from Apus, we are able 

 to assume that the first large locomotory limbs 

 must be homologous with the sixth pair, i.e. with the 

 first trunk limbs. The explanation of the great 

 differentiation of the first pair of trunk limbs, in all 

 these primitive Crustacea, is no doubt to be found 

 in the fact that the sixth segment was really the 

 first free segment, i.e. the first segment not used up 

 in any way in the bend which forms the head. Its 

 parapodia were thus free to develop as limbs for 

 locomotion or for some other function unconnected 

 with the mouth (sec Fig. i). The development of the 

 anterior trunk limbs into maxillipedes in the higher 

 Crustacea, has long been considered to be a secondary 

 modification. In the chief point which constitutes 

 them maxillipedes, i.e. in the retention of the ventral 

 parapodia as masticatory ridges, they are however the 

 more primitive form of limb. Those Crustaceans, on 

 the contrary, in which the first trunk limbs have lost 

 all traces of the ventral parapodia, and are purely loco- 



