168 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



limbs, are spoken of as if they were totally different 

 structures, under the name of branchiae or gills. 



The flagellum or epipodite of the first maxillipede, 

 however, is nothing but the slightly modified stem of a 

 podobranchia, which has lost its branchial filaments ; 

 but the term " ei^ipodite " may be conveniently used for 

 podobranchiffi thus modified. Unfortunately, the same 

 term is applied to certain lamelliform portions of the 

 branchiae of other Crustacea, which answer to the laminae 

 of the crayfishes' branchiae ; and this ambiguity must be 

 borne in mind, though it is of no great moment. 



On examining an appendage from that part of the 

 thorax which lies behind the third maxillipede, say, for 

 example, the sixth thoracic limb (the second walking leg) 

 (fig, 46), the two joints of the protopodite and the five 

 joints of the endopodite are at once identifiable, and so 

 is the podobranchia; but the exopodite has vanished 

 altogether. In the eighth, or last, thoracic limb, the 

 podobranchia has also disappeared. The fifth and 

 sixth limbs also differ from the seventh and eighth, 

 in being chelate ; that is to say, one angle of the distal 

 end of the propodite is prolonged and forms the fixed leg 

 of the pincer. The produced angle is that which is 

 turned downwards when the limb is fully extended 

 (fif^. 46). In the forceps, the great chela is formed in 

 just the same way ; the only important difference lies in 

 the fact that, as in the external maxillipede, the basipo- 

 dite and the ischiopodite are immoveably united. Thus, 



