xxvii APPENDAGES 325 



its inner edge, and bearing on its anterior border a little 

 three-jointed feeler-like body, the palp, the two distal seg- 

 ments of which represent the endopodite, its proximal 

 segment, together with the mandible proper, the protopodite. 



The antenna (2) is of great size, being nearly as long as 

 the whole body. It consists of an axis of five podomeres, 

 the fifth or last of which bears a long, flexible, many-jointed 

 structure, or flagellum (fl), while from the second segment 

 springs a scale-like body or squame (ex). It is fairly obvious 

 that the two proximal segments represent the protopodite, 

 the remaining three, with the flagellum, the endopodite, and 

 the squame the exopodite. 



The antennule (i) has an axis of three podomeres ending 

 in two many-jointed flagella (fl. i,fl. 2), which are some- 

 times considered as endo- and exopodite. But in all the 

 other limbs, as we have seen, the exopodite springs from 

 the second segment of the axis, and the probabilities are 

 that there is no exact correspondence between the parts of 

 the antennule and those of the remaining appendages. 



The eye-stalks, already noticed, arise just above the an- 

 tennules, and are formed each of a small proximal and a 

 large distal segment. They are sometimes counted as 

 appendages serially homologous with the antennae and 

 legs, &c., but are more properly to be looked upon as 

 articulated processes of the prostomium. It is possible 

 that the antennules are also prostomial and not metameric 

 structures : assuming this to be the case, it will be seen 

 that the body of the crayfish consists of a prostomium, 

 eighteen metameres, and a telson, which is probably com- 

 posed of an anal segment plus a post-anal extension. The 

 prostomium bears eye-stalks and antennules : the first four 

 metameres are fused with the prostomium to form the head, 

 and bear the antennae, mandibles, first maxillae, and second 



