chap, xxiii.] UNITS OF REPETITION. 557 



that the repetition may begin from any point in an appendage 

 and include all the parts peripheral to the point of origin. Seeing 

 that the repeated parts are, in their degree, comparable with a 

 whole organism, this indefiniteness is remarkable. We have thus 

 to recognize that the property of morphological "unity" may 

 attach not only to a pair of appendages beginning from the 

 body, or from some definite surface of articular segmentation, but 

 also to a pair of parts having no semblance of morphological dis- 

 tinctness. 



Strangest of all is the repetition of the index of Crabs and 

 Lobsters in Secondary Symmetry. The dactylopodite is of course 

 a separate joint. Double extra dactylopodites in Secondary Sym- 

 metry present no feature different from double extra tarsi, &c. 

 But the index we think of as merely a large spine or tubercle. 

 It is in no sense a joint or segment. Yet a pair of indices may 

 be added to a normal body. The interest of this fact is in its value 

 as a comment on the principle given on p. 476 that extra parts 

 in Secondary Symmetry contain the structures peripheral to their 

 point of origin. The case of extra indices shews that the term 

 peripheral, if it is to include the case of indices, must be inter- 

 preted as meaning not morphologically but geometrically peri- 

 pheral l . 



We have spoken of parts in Secondary Symmetry as having 

 no place in the Primary Symmetry of the body. This is on 

 the whole a true statement, but there are a few cases which 

 make it uncertain whether it is absolutely true. These cases 

 are those few where repetitions in Secondary Symmetry were 

 present on appendages of both sides of the body. 



Cases of this class were Odontolabis stevensii, No. 799, and 

 Melolontha hippocastctni, No. 795, where such extra parts were 

 present on both antennae, suggesting that the similarity of the 

 repetition of the two sides is due to the relation of Symmetry 

 between the right side and the left. But against this view may 

 be mentioned the cases Prionus cor'ia?*ius, No. 750, and Carabus 

 irregularis, No. 760, where two legs of the same side each bore 

 extra parts, and the Lobster, No. 821, having two pairs of extra 

 points on one dactylopodite. These cases suggest that bilateral 

 simultaneity in such repetition may perhaps represent merely 

 a general capacity for this form of repetition. The case of 

 Prionus californicus, No. 843, would no doubt bear on this 

 question, but unfortunately the facts in that case are scarcely well 

 enough known to justify comment. 



1 A case is given by Faxon (Harv. Bull., vin. PI. n. fig. 8) of Callinectes has- 

 tatus in which the left lateral horn of the carapace, instead of being simple as in 

 normal specimens, had three spines. It is just possible that two of these may have 

 been in Secondary Symmetry. All other cases known to me are in appendicular 

 parts. 



