2. Effect on the primary and secondary sexual characters. g7 



females will be compared with these characters in uninfected individuals of either sex. Since 

 the crab does not moult again after once the parasite has become external there is no pro- 

 gressive effect on the secondary sexual characters after this state has been assumed. Under 

 the heading B the condition of the sexual gland is dealt with. In C those crabs are studied 

 which have been so profoundly modified both externally and internally that it is difficult to 

 assign them for certain to either sex, though, as we shall see, these crabs are almost certainly 

 profoundly modified males. In D the highly unexpected phenomena exhibited by recovered 

 crabs are described. 



A summary of the chief results and a general discussion are reserved until certain facts 

 relating to other forms have been considered. 



A preliminary point must be settled relatively to the time at which the transformation 

 of the secondary sexual characters occurs. The parasite becomes external, after establishing 

 its root system in the host, by means of an ecdysis of the latter; it is at this ecdysis that 

 the external transformation due to the parasitic castration invariably occurs, and from this point 

 onward, owing to the inhibition of growth, the host does not moult again. In about thirty or 

 forty cases I have observed that this moult has failed to bring the Sacculina to the exterior, so 

 that the phenomenon was observed of the effects of parasitic castration being apparently brought 

 about by Sacculina interna. But these cases, all of them in large crabs with thick chitinous 

 integuments, are only peculiar exceptions easy of recognition, because the Sacculina interna 

 was evidently at a period of development when it ought to have been evaginated. In normal 

 cases therefore the effects of parasitic castration are revealed at the same period in the life 

 history of the parasite, and do not alter again after that period, and it is owing to this fact 

 that the phenomena of parasitic castration in these crabs afford us so secure and easily appre- 

 ciable a means of analysing the sexual characters of infected animals, since the cause of the 

 castration is always equivalent and the same, so that the differences in the effect must be 

 chiefly due to differences inherent in the infected animals. 



A. Effect on the secondary sexual characters. 



1. Females. The normal uninfected females of /. scorpio occur under two sharply 

 differentiated forms which I call adolescent and adult. The adolescent crabs (Plate 7 

 figs. 1 and 2) have flat and comparatively small abdomina and the abdominal appendages are 

 short, stout, and unprovided with long hairs. This adolescent form is preserved by the 

 female until the first brood of eggs is produced, that is at any rate till the female measures 

 12 millimeters in carapace length, and very frequently later, especially in winter when no 

 reproduction is occurring. 



The adult females on the other hand (Plate 7 figs. 3 and 4) have large trough-shaped 

 abdomina with four long biramous appendages, provided with long hairs, the endopodites being 

 furnished with exceedingly long branched hairs which carry the eggs. These appendages are 



