REGENERATION IN CAMBARUS. 305 



lack of careful observation ; but the examinations which have 

 been made show that they occur infrequently. 



The abnormalities presented here are of interest for several 

 reasons. The abnormal pleiopod is evidently the result of a 

 regeneration after extensive injury in one instance, and just as 

 evidently the result of undisturbed growth processes in the other 

 instance. That is, the condition is congenital and may be con- 

 sidered as having arisen by mutation. 



It would appear that the swimmerets shown in Figs, i and 2 

 probably resulted from an injury which removed part of the 

 pleuron, and tore the first and second pleiopods near the base of 

 each, as represented diagrammatically in Fig. 17. The coxop- 

 odite in Fig. I is abnormally large, which condition may pos- 

 sibly be explained by the fact that a large area was exposed 

 when the mutilation occurred. 



Very few genuine cases of duplicity have been described. 

 Bateson mentions only four cases, all chelae ; Herrick has figured 

 a double chela of a female lobster, and Zeleny has described a 

 double chela of Gelasiinns pugilator which regenerated instead of 

 a normal single one during the course of his experiments. 



Bateson states emphatically that " in arthropods and verte- 

 brates 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 right, and no 

 crustacean has two right legs in succession where one should be." 



While this supernumerary appendage may be regarded as a 

 complementary image of the normal one, and therefore a left 

 appendage instead of a right, there is nothing in the structure of 

 either member to indicate that such a relation exists. The 

 members are not imperfect, and are placed in succession, differ- 

 ing in these two respects from the cases admitted by Bateson. 



At first appearance the coxopod of the abnormal first append- 

 age is very much like that of the last pereiopod. And the an- 

 terior uniramous part of the second pleiopod is like the first 

 pleiopod in being uniramous, but is a typical endopodite. There 

 is a mere suggestion here of a shifting backward of the series of 

 organs, a condition to which Bateson applied the term backward 

 homceosis. 



