526 MERISTIC VARIATION. [part i. 



Of the whole number, two affect antennas, four are in non- 

 ehelate ambulatory legs, one is in a chelate ambulatory leg and 

 the rest, being the great majority, are all in chelae. 



With reference to these extra parts several false views have 

 from time to time been held. For example, in some of the 

 commonest cases there is an extra pair of dactylopodites, or of 

 indices, curving towards each other. The extra parts may then 

 greatly resemble the dactylopodite (or " pollex ") and index of 

 a normal chela, and many authors have not unnaturally supposed 

 that the extra parts were actually an extra pair of forceps re- 

 peating those of the normal chela. This may easily be shewn 

 to be an error, from the fact that it is often possible by some 

 slight structural difference between the pollex and the index to 

 detect that both extra parts are either both pollices or both 

 indices. 



But the fullest disproof of this supposition is found in the 

 fact that the great majority of the phenomena will be readily 

 seen to conform to the principles enuntiated for Secondary Sym- 

 metries in Insects (p. 479). 



A good many authors from the time of Rosel vox Rosexhof 1 

 onwards have said that these cases are a result of injury, or of 

 regeneration after injury. For this belief I know no ground. 

 It should be remembered as an additional difficulty in the way 

 of this belief, that when the limb of a Crab or Lobster is injured 

 it is usually thrown off bodily, while the extra parts most often 

 spring from the periphery of the chela. But since, according to 

 Heixekex 2 , such mutilated parts are sometimes retained, this 

 must not be insisted on. 



In the case of an ambulatory leg the surfaces may be named 

 as in an insect (without any suggestion that these names denote 

 true homologies between the surfaces so named). In describing 

 chelae I propose to use the following arbitrary terms. The border 

 upon which the dactylopodite articulates is the pollex-border, the 

 opposite border being the index-border. It should be noted that 

 in the Crab the pollex-border is superior, but in a Lobster 3 it 

 is internal. 



(1) Clear cases of Extra Parts in Secondary Symmetry. 



A. Legs. 



*808. Palinurus vulgaris : left penultimate ambulatory leg bore 

 two supernumerary legs (Fig. 180). Coxopodite of great width. 

 The basipodite had three articular surfaces as shewn in Figure 180, 



1 Eosel von Rosenhof, Insekten-Belustigung ', 1755, in. p. 344. 



2 Heineken, Zool. Jour. 1828—29, iv. p. 284. 



3 It is worth noticing that in the chela of a Scorpion though a close copy 

 of that of a Decapod, the arrangement is reversed, the articulated pincer being 

 external. 



