332 Prof. A. Giarcl on Parasitic Castration 



not project so much laterally beyond the following segments, 

 Segment 6 (that which in the Macrura bears the uropods) 

 is rounded on its free margins and presents nearly the form of 

 a semicircle instead of being trapezoidal as in the ordinary 

 male. 



All these modifications are produced in a more or less com- 

 plete fashion according as the crab has been infested at a more 

 or less advanced age ; old males bearing SacmdincB do not 

 differ at all from normal males, or, at the utmost, segment 6 

 is slightly dilated at its margins. 



I have observed modifications exactly identical with those 

 of Carcinus mcenas in young males of Portunus holsatus in- 

 fested by Sacculina Andersonii, sp. n. (fig. VIL). 



The males of Cancer pagurus infested by Sacculina trian- 

 gularis^ Anderson, those of Portunus puher^ infested by 8. 

 Priei^ sp. n., and those of Platyonychus latipes, infested by 

 S. Betencourtij sp, n., present much less considerable modifi- 

 cations. 



III. 



My attention having thus been attracted to the influence of 

 parasitic castration, due to the presence of a Rhizocephalan, I 

 examined more carefully than I had previously done the male 

 Crustacea infested by the Isopods of the group Bopyridse. I 

 knew that these Isopods usually cause the sterility of their 

 hosts. However, there are more numerous exceptions than in 

 the case of the Rhizocephala, and in not a few instances we 

 can still observe some activity, imperfect it is true, of the 

 male or female genital glands of the Decapods bearing Ento- 

 niscus or Bopyridse. 



The young males of Carcinus mce^ias infested by Portunion 

 mcenadis frequently present a modification of the external 

 sexual characters, but this modification is less profound than 

 that which we have considered above. It consists, in the first 

 place, of the less complete coalescence in the infested male of 

 the abdominal segments 3,4, and 5, and especially in the form 

 of the sixth segment, which becomes dilated at its free mar- 

 gins as in the Sacculiniferous males (see fig. IV.). This latter 

 peculiarity is very interesting. In the males bearing Saccu- 

 li72(e, the tail being gradually raised by the parasite, it might 

 be thought that the dilatation of the sixth somite was due to 

 the circumstance that this segment was no longer confined 

 within the groove hollowed out in the sternal part of the 

 carapace of the crab. But the same explanation cannot be 

 given in the case of the male bearing Entoniscus, and one is 

 lorced to assume that this modification of the sixth somite is 



