CHAP, xxi.] SECONDARY SYMMETRY : CRUSTACEA. 



535 



than the last. The cases known to me amount to about ten 

 or fifteen. Seven cases are illustrated in Figs. 189, 190, and 191. 



I. 



FIG. 191. Two cases of extra indices in Cancer pagurus. I. In Coll. Surg. 

 Mus. II. After le Senechal. R, normal right index. L, normal left. R', L', 

 extra rights and lefts. 



(d) Simple processes, probably being rudimentary extra pairs 

 of indices or of dactylopodites. 



*824. Many such are described, but of 

 few can anything be said with confid- 

 ence. A comparatively simple case 

 is shewn in Fig. 192, where there is 

 a decided suggestion that the process L' 

 + R' is morphologically a pair of indices 

 that have not separated from each other 

 but stand compounded by their toothed 

 borders. On comparing this case with 

 for instance, Fig. 191, II, it will be seen 

 that the two conditions might readily 

 pass into each other in the way so often 

 seen in Insects. 



Other cases of a more doubtful cha- 

 racter are shewn in Fig. 193. Though 

 in each the nature of the extra part is 

 obscure, it is probable that they are all 

 rudimentary states of the repetitions 

 described. The alternative view that 

 they are single repetitions certainly can- Fio. 192. Left chela of Portu- 



not be applied to all, for in many the U J IS P" bc >' { J om ^ SENECHAL, Hull 



, . ,, ',,., .Soc. zoo/, trance, 1888, xin. p. 125. 



i-xtra process, though in the plane of the L> normal illdex L , + /r % pah . 



index and dactylopodite, is similar on of extra indices in Position V. 



