description of an abnormal lobster cheliped. 265 



Cause of the Abnormality. 

 While believing that we must ultimately look to experiment 

 for the elucidation of the causes of abnormalities of this sort, 

 there are nevertheless certain considerations in the present in- 

 stance which are of interest. On the theory that they may be 

 the result of injuries, marks indicating such injury have often 

 been sought. The scar of an evident injury is very notice- 

 able on the carpopodite of the present specimen (Fig. 3, s.); 

 but is is equally evident that this injury cannot have been the 

 the cause of the abnormality, since, as we have seen, the effect 

 of the doubling is obvious as far proximad as the ischiopodite. 

 It seems more probable that after some antecedent autotomy of 

 the appendage the regenerating bud was in some way injured or 

 disturbed so as to produce the abnormality. A definite scar 

 for such an injury could not be expected in the fully developed 

 appendage. Not only would it be of the greatest importance 

 to know whether this abnormality would be reproduced if the 

 leg could have been amputated autotomously and allowed to 

 regenerate (cf. Emmel, 1907, p. iii), but it would also be in- 

 teresting to know whether a lobster could moult normally with 

 an appendage of this sort. On account of the extra amount of 

 material involved, which would have to be drawn through the 

 narrow opening at the base of the leg, it would certainly be a much 

 more precarious ordeal than usual. 



Summary. 

 In this paper is described the abnormal cheliped of a lobster 

 {Homarus americanus) , the abnormality consisting of a double 

 extra claw. While the extra part actually separates from the 

 normal propodus, it is shown that the effects of the doubling may 

 be traced to the base of the leg. It is further shown that the 

 conditions in this appendage illustrate almost diagrammatically 

 the "rules of secondary symmetry" formulated by Bateson, if 

 allowance be made for the effects of the torsion which occurs 

 in the normal lobster cheliped and the mechanical conditions 

 which may modify such torsion. 



