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[FRoM THE AMERICAN JOURNAL OF SCIENCE, Vou. XXVII, January, 1884. |
Arr. VIII.— On the so-called Dimorphism in the Genus
Cambarus ; by WALTER Faxon,
THE existence of two forms of the adult male in all the
species of the genus Cambarus was discovered by Louis Agassiz
and Henry James Clark. The differences between the two
forms affect more especially the first pair of abdominal append-
‘ ages, organs concerned in the act of coition, but also extend to
the general form and sculpture of the body. In one form (un-
happily called by Dr. Hagen the “second form ”), the first pair
of abdominal appendages have a structure nearly like that seen
in all young males. The hooks on the third joint of the third
(in some species of the third and fourth) pair of legs are small,
and in the scuipture of the shell and shape of the claws, this
form approaches the female. In the other form (Hagen’s “first
form”), the articulation near the base of the first pair of abdom-
inal appendages is gone and the whole member is much more
highly specialized, the terminal hooks being horny, more
widely separated and in every way more highly developed; in
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
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 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 alcohol 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 exuviz 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 pairing 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 propin-
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 Miller (Fiir Darwin) has pointed out the existence of
two forms of the male in the genera Zanais 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.
44. W. Faxon—Dimorphism im the Genus Cambarus.
Such a change as this connected with the reproductive
periods is unparalleled, so far as I know, among the Inverte-
brata; and even among the Vertebrata¥ the cases of partial
atrophy of the generative organs or shedding of antlers (as in
the stag) after the rut is over are hardly comparable.
‘At the time I had the specimens alive my attention had not
been drawn to the questions relating to the two forms of the
males, so that I failed to make anatomical examination, and
the specimens have now lain too long in alcohol to be service-
able for internal dissection. I hope, however, that naturalists
who are more favorably situated will be able to throw more
light on this subject.
I will add that the males of extraordinary size which I have
seen, are all of the “first form.” Do these very old individuals
cease to moult? Do they become permanently capable of re-
production ?
Museum of Comparative Zodlogy, Cambridge, Mass., Nov. 12, 1883.
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