PUZZLES OF PARASITISM 109 
be regarded as genera later on, and might therefore just as 
well be called so at first. But for his change of the 
specific names de Haan had a reasonable though a rather 
singular excuse. After his plate had been engraved with 
the figures and names of two distinct species, based on 
the very great dissimilarity of the carapace in the male 
and female specimens at first available, he received a new 
series of specimens belonging to each form. These he 
found to agree in so many particulars that the protean 
form of the carapace no longer sufficed to separate 
them specifically, but had to be regarded as depending on 
the variations of age or sex. He here also observed that, 
as in several other Crustacea, there were what he supposed 
to be two forms of the female, the one ovigerous which he 
designates as genuine, the other sterile which he speaks 
of as spurious. In the present species he says that the 
pleon in the genuine females is of five segments, the fourth 
thick, very convex, with a longitudinal median impression, 
while in the spurious females it has seven segments, is 
lamellar and concave, with the fourth, fifth, and sixth 
segments dilated, the seventh narrower and truncate. 
The mystery of these so-called spurious females has been 
recently explained by Professor Giard. In studying the 
parasitic Crustacea, as well Isopoda as Rhizocephala, he 
was confronted by the statement which Rathke made in 
1837, that of the many hundreds of Bopyrus which had 
passed through his hands he had never found one in any 
but a female Palemon, and also by the statement which 
Fraisse made in 1877, that the males of Jnachus, so far as 
he had observed, were never attacked by parasites, which 
he thought might be the result of the unsuitable shape of 
the narrow pleon in that sex. i 
The parasitic groups will be for after consideration. 
but it must here be mentioned that by careful investiga- 
tion Professor Giard arrived at the very interesting result 
that the specimens of the higher Crustacea infested by the 
parasites are as a rule more or less completely sterilised, 
and that the secondary sexual characters are considerably 
modified, so that the males acquire to some extent the 
