TYPE CRUSTACEA. 



373 



manner tlie grasping claws (Fig. 165, B) or chelae are devel- 

 oped by the flexion of the terminal joint on the subterminal or 

 by the elongation of the angles of the latter into a more or 

 less strong process against which the terminal joint may be 

 approximated. 



The description given above of the various appendages is 

 of course general, the modifications found in the various 

 forms being almost endless. Indeed in parasitic forms the ap- 

 pendages, except those concerned in mastication, may entirely 

 disappear, all gradations between fully-developed append- 

 ages and the merest rudiments being found in various forma. 



Fig. 166.— Sixth {A) akd Sbcond (5) Thoracic Appendages op Bkanchio- 



POD, Apus (after Zadsacb from Bronn). 

 br = bract. fl = flabellum. 1-6 = inner lobes. 



From what has been said, however, it may be seen that typi- 

 cally the Crustacean appendage may be considered a biramous 

 structure, consisting of a two-jointed basal portion termed 

 the protopodite and two jointed branches termed the exopodite 

 and endopodite (Fig. 165, ex, en) according to their relation to 

 the median axis of the body. Additional rami are frequently 

 developed upon the protopodite — such, for example, as that 

 termed the epipodite (Fig. 167, ep) and the branchia (br). How- 

 ever, although such a limb may be considered typical, it is 

 not necessarily also the most primitive. Indeed when the 

 simplest forms, such as the Phyllopoda, are examined it will 

 be found that the more posterior appendages have a very 

 different composition. Thus in the genus Apus the sixth 

 thoracic appendage (Fig. 166, A) consists of a central two- 



