52G M ERISTIC VARIATION. [PART i. 



Of tin- whole numliiT, two affect antennae, four are in uon- 

 chelate ambulatory l'-gs, one is in a chelate ambulatory leg and 

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



With reference to these extra parts several false views have 

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

 ( oiniiioiit --t r.i>cs there is an extra pair of dactylopodites, or of 

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

 greatly n--<'inble the dactylopodite (or "pollex") and index of 

 ;i normal chela, and many authors have not unnaturally supposed 

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

 |n-ating tlmse of the normal chela. This may easily be shewn 

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

 slight struetural 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 tn conform to the principles enuntiated for Secondary Sym- 

 metries iu Insects (p. 479). 



A good many authors from the time of ROSEL VON RosENHOF 1 

 in wards 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 

 HEINEKEN", such mutilated parts are sometimes retained, this 

 muM not lie insisted on. 



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

 as in an insect (\\ithout any suggestion that these names denote 

 true homolugies 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 

 i.|ip.i>ite border being the i mles-border. It should be noted that 

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

 is 



( I i Clear cases /' /'.// /*/ l'<n-ts in 

 A. Legs. 



*S(),s. Palinurus vulgarise left penultimate ambulatory leg bore 

 two supernumerary legs (Fig. IM)). ( 'oxopodite of great width. 

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



1 ROM i. vi IN I;SI:MII>I , Insekten-Belustigung, 1755, in. p. 344. 



' III IM KIN. ZOOl. -'"lir. IS'JS '.".I, IV. p. -JS|. 



1 It is \\orlh untiring that in tin- rh, la of u Scorpion though a close copy 

 of that of a Decapod, tin 1 an-aiignm-nt i> reversed, the articulated pincer being 

 external. 



