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 Palcemon, and also by the statement which 

 Fraisse made in 1877, that the males of Inachus, 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. 



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 



