ill the Decapod Crustacea. 329 



and all the external characters of the female appendage, and 

 seemed to be arranged to protect the parasite as perfectly as 

 it protects the eggs in the other sex. 



Moreover, the secondary sexual characters of these infested 

 males were likewise modified in the same direction as the 

 primary characters. The chelaj of the first pair of legs, in- 

 stead of being strongly developed and projecting far beyond 

 the head as in the normal males, were feeble and reduced as 

 in the female sex. All these peculiarities are the more striking 

 as in its ordinary state Stenorhynchus is one of the Brachyurous 

 Decapods in which the sexual dimorphism is most accentu- 

 ated. A drawing of these males, castrated by the parasite, 

 eeems to be absolutely useless, as it would be confounded with 

 the classical figures given of the female sex. The number of 

 these males, moreover, is more restricted than that of the 

 females (about one to six according to my statistics). lu 

 presence of this result I have QVQxy reason to think that 

 Fraisse, being more particularly engaged in the investigation 

 of Cryptoniscus^ contented himself with a too rapid examina- 

 tion of the Inachus scorpio infested by Sacculina neglecta^ 

 and that in this Oxyrhyuchan, as in Stenorhynchus^ the male 

 sex is not free from tlie attacks of the Ehizocephalan, 



IL 



Since 1873, I may say without exaggeration that thou- 

 sands of Carctnus mcenas bearing Sacculince have passed 

 under my eyes. More recently M. Yves Delage, on his part, 

 has examined a respectable number of these animals. He has 

 done this with a very legitimate but particularly lively desire 

 to see something that 1 had not seen. In other countries Koss- 

 mann has also studied the Rhizocephala with much care and 

 success. Nevertheless neither Kossraann, nor Delage, nor 

 myself had noticed a very important fact, which one cannot help 

 seeing when, instead of looking^ one observes. This fact may 

 be enunciated as follows : — When a young male Carcinus 

 ma^-nas is infested hy a Sacculina it acquires, in part^ the ex- 

 ternal sexual characters of the female sex. The resemblance 

 may be carried so far as to cause for a moment a difficulty in 

 the determination of the sexes if we neglect to lift the 

 caudal appendage. 



Even last year, when I announced to the Academy of 

 Sciences the curious observations which I had made upon the 

 Sacculina of Stenorhynchus *, I still regarded that type as 

 exceptional, and thought that in most of the Decapod Crusta- 



* Comptes Rendus, July 5, 1886, p. 84 ; ' Annals/ ser. 5, vol. xviii. 

 p. 165. 



