148 Miscellaneous. 
all of the “ second form,” their exuvice of the “first form”! After 
attaining the “first form” and after pairing 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 indi- 
vidual, 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 repro- 
duction 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 propinquus, 
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 Yanais 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 Cam- 
barus. 
Such a change as this connected with the reproductive period is 
unparalleled, so far as I know, among the Invertebrata, and even 
among the Vertebrata ; the cases of partial atrophy of the genera- 
tive 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 serviceable for internal 
dissection. I hope, however, that naturalists who are more 
favourably 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 reproduction ?— 
Amer. Journ. Sci., January 1884, p. 42. 
Museum of Comparative Zoology, 
Cambridge, Mass., 
Noy. 12, 1883. 
On Visual Organs in Solen. By Dr. B. SHarpe. 
Dr. Benjamin Sharpe called attention to a remarkably primitive 
form of visual organ that he had discovered in the siphon of Solen 
ensis and S. vagina (the common “ razor-shell ”’). 
His attention was directed to the probable possession of visual 
