W. Faxon — Dimorphism in the Genus Cmiibarus. 43 



those species with bifid tips to these appendages, the branches 

 are longer, slenderer, more widely separated and stiffer; the 

 hooks on the thoracic legs are longer and more perfectly finished ; 

 the sculpture of the whole body is more pronounced and the 

 claws are larger and more powerful. No intermediate condi- 

 tions are found, and there is no relation between these forms 

 and the size of the individual, the "second form" being large 

 and the "first form" small, or vice versa. Hence we are for- 

 bidden to interpret the two forms as stages in ordinary devel- 

 opment. Dr. Hagen has shown that in individuals of the 

 " second form " the internal generative organs are smaller than 

 in the "first form," but having only alcoholic material he was 

 unable to determine anything concerning the presence or ab- 

 sence of spermatozoa. He interprets the facts as a case of 

 dimorphism and surmises that the "second form" males are 

 sterile individuals. 



In the autumn of 1875, I received a lot of living Gamharus 

 rusticus Girard, frojn Kentucky, males of the " first form " and 

 females, which bred freely in confinement. After pairing, three 

 of the males moulted and were thrown, while in the soft-shelled 

 state, into alcohol together with their exuviae. An examina- 

 tion of these specimens now reveals the fact that the soft-shelled 

 specimens are all of the "second form," their exuviae of the 

 " first form !" After attaining the "first form " and after pair- 

 ing, the same individual has reverted to the "second form." 

 It is now clear that we are not dealing with a case of true 

 dimorphism such as is well known among insects and plants, 

 but it appears probable that the two forms of the crayfish are 

 alternating periods in the life of the individual, the " first form " 

 being assumed during the pairing season, the "second form" 

 during the intervals between the pairmg seasons. It is to be 

 inferred that before the animal is again capable of reproduc- 

 tion, another moult will bring it again into the " first form." 



The fact that large collections, made at one time and place, 

 often contain only one or a great preponderance of one, form of 

 the male, is now explained. 



I have also before me a male specimen of Cambarus pro2nn- 

 quus Girard, from Wisconsin, belonging to the Peabody Museum 

 of Yale College, which was taken in the act of moulting. The 

 old shell is "first form," the soft shell emerging from it is 

 " second form." 



It is remarkable that two forms of the male have not been 

 detected in any other genus of crayfishes. 



Fritz Miiller (Fiir Darwin) has pointed out the existence of 

 twd forms of the male in the genera Tanais and Orchestia which 

 be considers as truly dimorphic forms. It is possible that these 

 are to be explained in the same way as the two forms of the 

 male Cambarus. 



