CHAPTER XXI. 



APPENDAGES IN SECONDARY SYMMETRY continued. 



THE EVIDENCE AS TO CRUSTACEA 1 . 



THE facts as to Secondary Symmetries in Crustacea are so 

 similar to those already detailed in Insects that, were it not for 

 their value as confirmation of the principles indicated, it would 

 be scarcely necessary to describe them at large. Some few of 

 the cases have besides a special interest, as in them may be seen 

 rudimentary or bud-like structures apparently presenting the 

 lowest condition of paired parts in Secondary Symmetry. 



Precisely as in Insects there are a number of cases (including 

 those last mentioned) where it would at first sight be supposed 

 that the extra parts are single, but on inspection most of them 

 prove double. Nevertheless there remain some few where this 

 cannot be shewn, and strange -as it may seem, these must be 

 admitted to be genuine examples of duplicity of limbs. Of them 

 a special account will be given in another chapter. 



There are besides, as in Insects, a considerable number of 

 cases in which the nature of the parts is not clear, though the 

 majority of such cases are not examples of extra parts, but art- 

 normal appendages mutilated or deformed. 



One specimen (No. 821) is the only case known to me in 

 which two pairs of supernumerary parts arise from one append- 

 age. 



Another (No. 827) is unique in the fact that according to the 

 description three separate appendages are repeated upon a single 

 appendage. It is not clear that this is in any strict sense an 

 instance of Secondary Symmetry, but for convenience it is taken 

 in this chapter. 



1 Useful bibliography given by FAXON, Hun-. Hull., 1880 1, viu. p. 271. 



