330 Prof. A. Giard on Parasitic Castration 



cea the atrophy of the male genital glands, caused by the 

 presence of a SaccuUna, was unaccompanied by any modifi- 

 cation of the external sexual characters. Nevertheless I had 

 a vague recollection of the embarrassing cases to which I have 

 alluded above. Hence, on my arrival at Wimereux during 

 the vacation, profiting by the circumstance that the Sacculina 

 was comparatively abundant that summer, I examined a great 

 number of infested specimens of Carcinus mcenaSj and I soon 

 found it easy to recognize, and to demonstrate to my pupils, 

 the effects of parasitic castration upon the young male crabs. 

 So true is it that, as Marey says, " observation is much more 

 intense and much more fruitful when the investigator knows 

 beforehand what he ought to find, and strives pertinaciously 

 to find it, notwithstanding want of success at the outset." 



The external sexual characters of the Brachyurous Decapod 

 Crustacea are too well known for us to delay much in describ- 

 ing them. It is well known that the principal one is that 

 the tail (abdomen) of these animals is generally broad and 

 oval in the female, while it is narrow and trapezoidal or tri- 

 angular in the male sex. This abdomen is composed of 

 seven somites, of which the first two (1 and 2) bear the 

 copulatory styles in the male, while in the female the somites 

 2, 3, 4, and 5 are furnislied with the plumose feet destined to 

 support the ova. Lastly, in a certain number of species, 

 notably in Carcinus mcpnas and in Portunus^ the somites 3, 4, 

 and 5 are intimately soldered together in the male in such a 

 way that the tail appears to be formed only of five segments, 



thus, 1, 2, 3,4^5, 6, and 7. 



Grobben has pointed out that the coalescence of the seg- 

 ments 3, 4, 5 in the male sex does not occur in certain Cyclo- 

 metopa {Eriyliia spinifrons^ Pihimnus lnrtellus\ and that even 

 in other groups of Brachyura (Notopoda, Oxystomata, Oxy- 

 rhyncha, Catometopa) side by side with forms in which there 

 are five segments in the abdomen of the male we find others 

 which have retained the seven primitive somites. Whence 

 it may be concluded that the coalescence has been produced 

 independently in the various sections of the Brachyura, and 

 that it probably constitutes an arrangement serviceable in the 

 act of copulation. Lastly, the chelas are generally more de- 

 veloped in the male sex. 



All these external sexual characters disappear more or less 

 when the crab is rendered sterile by the presence of a parasite ; 

 the copulatory styles and the ovigerous feet are frequently 

 more or less atrophied, but always much less so than in 

 Stenorhynchus. The modification bears especially upon the 

 general form of the tail, which in the male sex takes on the 



