CHAPTER XXII. 

 Duplicity of appendages in Arthropoda. 



That there should be such a thing as a limb double in the 

 sense in which the following are double, has always seemed to 

 me most strange. We know that a segment of an Annelid, or 

 a vertebra, may be on one side of the body divided to form 

 two segments or two vertebrae (as in No. 88 or No. 7) while 

 on the other side of the middle line the segment is single. This 

 is in keeping with all that we know of Division of parts in Linear 

 Series. So might we suppose that a parapodium, or a rib, or 

 perhaps a limb-bud might divide into two ; but the two half- 

 segments or half-vertebras are in Succession to each other, and 

 are not complementary images of each other as these double- 

 limbs are. 



That a parapodium may divide into two Successive para- 

 podia is possible enough, though, apart from division of the 

 segment bearing it, I know no clear case. But it may be stated 

 at once that in Arthropods and Vertebrates such a phenomenon 

 as the representation of one of the appendages by two identical 

 appendages standing in Succession is unknown. No right arm 

 is ever succeeded on the same side of the body by another arm 

 properly formed as a right, and no Crustacean has two right 

 legs in Succession, where one should be. The only cases at all 

 approaching this state are those of Macacus No. 504 (q. v.), a case 

 that must be interpreted with great hesitation ; and of the Frogs 

 described by Cavanna and by Kingsley, also doubtful cases 

 (see Chapter xxm). 



But though such repetition is probably unknown and is perhaps 

 against Nature, there are still these strange double-limbs : two 

 limbs, always I believe imperfect, placed not in Succession, but as 

 complementary images of each other, more or less exact. These 

 we have seen in the hand of Man and in the feet of Artiodactyles ; 

 we have now to study them in Insects and in Crustacea 1 . 



1 With mistrust I name cases in Amphibia and Fishes, perhaps of this nature. 

 Ziissotriton punctatus (Newt) : left pes having 10 digits in two groups, 6 and 4. 

 Coll. Surg. Mm., Ter. Ser.. 293, a [not dissected]. Protopterus annectens : rt. 



