W. Faxon—Dimorphism in the Genus Cambarus. 48 
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 
sence of spermatozoa. He interprets the facts as a case 0: 
dimorphism and surmises that the “second form” males are 
sterile individuals. 
n the autumn of 1875, I received a lot of living Cambarus 
rusticus Girard, from 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 aleohol together with their exuvie. An examina- 
tion of these specimens now reveals the fact that the soft-shelled 
Specimens are all of the “second form,” their exuvie of the 
“first form!” After attaining the “first form” and after pair- 
ing, the same individual has reverted to the ‘‘second form.” 
t 1s now clear that we are not dealing with a case of true 
dimorphism such as is well known among insects and plants, 
ut it appears probable that the two forms of the crayfish are 
alternating periods in the life of the individual, the “ first form” 
” 
. 
cing assumed during the pairing season, the “second form 
often contain only one or a great preponderance of one, form of 
the male, is now explained. 
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 1s 
‘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 
two forms of the male in the genera Tanais and Orchestia which 
he 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. | i 
