THE CRAYFISH 123 



that the endopodite and exopodite represent the fifth and sixth 

 endites. The epipodite represents the exites, and more prob- 

 ably the flabellum than the bract, the latter structure having 

 disappeared altogether. The transition to the second maxilli- 

 ped shows how the four proximal endites are reduced, and 

 the joints of the axis which bore them become the coxopodite 

 and basipodite ; the fifth endite being as before the endopodite 

 and the sixth the exopodite. A similar comparison shows the 

 homologies of the rest of the series. Thus we are able, by 

 means of the graduated change of form and of relative size of 

 parts in the foot-jaws of the crayfish to trace the connection 

 between such apparently different structures as the chelae or 

 walking limbs of a decapod and the foliaceous appendages of 

 a phyllopod. 



To return to the remaining appendages of the crayfish. The 

 first maxilla (fig. 27, IV) is much reduced: the coxopodite 

 and basipodite are there, as two broad setose plates, the 

 endopodite (fifth endite) is also there, but is small and un- 

 jointed, the exopodite (sixth endite) and epipodite have 

 disappeared altogether. In the mandible (fig. 27, ///) the 

 protopodite is very large and transversely elongated, its inner 

 edge being divided by a groove into two toothed ridges. The 

 endopodite or fifth endite is represented by a three-jointed 

 appendage of the protopodite, usually known as the palp, and 

 it should be noticed that, in this respect, the mandible of the 

 crayfish does not attain to the same degree of differentiation 

 as that of Apus. In the second antennae of the crayfish 

 (fig. 26, //) we can recognise protopodite, exopodite, and 

 endopodite. The coxopodite is small, but furnished with a 

 large tubercle on its lower surface, upon which the opening 

 of the green gland is situated. The basipodite is divided by 

 a deep longitudinal groove into two pieces, and bears at its 

 distal extremity the expanded lancet-shaped squame, repre- 

 senting the exopodite, and the long feeler, representing the 

 endopodite. The feeler has two stout basal joints, succeeded 

 by a very long many-jointed filament beset with setse. 



The first antenna (fig. 26, /) consists of a large and 

 peculiarly shaped proximal joint succeeded by two shorter 

 joints ; the distal one bearing two terminal many-jointed 

 filaments, of which the outer is thicker and longer than the 

 inner. The outer filament bears two groups of curiously 



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