560 JOURNAL OF SCIENCE. 
POLYMORPHISM AMONG THE AMPHIPODA.* 
+ 
BY CHAS. CHILTON, M.A, 
> 
Fritz Miiller appears to have been the first to call attention to 
examples of dimorphism among the smaller crustacea. In his “Facts 
for Darwin” he gives two instances, one in the Isopoda and one in 
the Amphipoda. The first one is the now well-known case of Tanais 
dubius Ky.; in this species, according to Fritz Miiller, the males 
resemble the females until the last change of skin before attaining 
sexual maturity, but after that moult they appear under two different 
forms between which there are no intermediate varieties. ‘Some 
acquire powerful, long-fingered, and very mobile chele, and instead of 
the single olfactory filament of the female, have from 12 to 17 of these 
organs, which stand two and three together on each joint of the 
flagellum. The others retain the short thick form of the chele of the 
females; but, on the other hand, their antenne are equipped with a 
far greater number of olfactory filaments, which stand in groups of 
from five to seven together.t Fritz Miiller’s other example is from 
the genus Orchestia; in Orchestia Darwinii there are two forms of the 
male which differ from one another in the shape of the second pair of 
gnathopoda to such an extent that they would without doubt be 
considered as two distinct species were they not found living on the 
same spot with undistinguishable females, Mr. G. M. Thomson has 
recorded a similar phenomenon in the species Orchestia sylvicola which 
may be found in damp places under dead leaves and among roots of 
grass, &c., all over New Zealand. The males are extremely rare, I 
have seen only one though I have examined scores of specimens. In 
the “Transactions of the New Zealand Institute,’, Vol. XIII. p. 212, 
Mr. Thomson gives the results of a detailed examination of a large 
number of specimens gathered from many localities and comes to the 
conclusion that they all belong to one species “the males of which 
have at least two forms of gnathopoda.” 
In many genera of the Amphipoda such as Mera, Melita, Para- 
nenia, Podocerus, Orchestia, &e., the females of the different species 
are often much alike while the males which usually have one pair of 
gnathopoda abnormally developed may differ considerably from one 
another. In these cases, therefore, if a supposed instance of dimor- 
phism should occur, it is extremely difficult to decide whether it is 
really so or whether there are simply two species of which the males are 
different but the females nearly alike; and, as Fritz Miiller points out, 
it is only in those cases where the species can be procured in sufficient 
abundance that we can really decide. There are doubtless many 
instances beyond those that have been already recorded, but, owing 
to the want of sufficient evidence, they are at present unrecognised. 
On the other hand it will not do to be hasty in jumping to the con- 
*A paper on the same subject, but entering more fully into technical detail, and 
illustrated with figures, has been sent to the ‘“‘Annals and Magazine of Natural 
History.” —C. C. 
+ ‘‘Facts for Darwin,” pp. 20, 21. 
