C9 The effect of the Parasites on their hosts. 



5. General Conclusions and Summaries. 



Before entering into a general discussion on the subject of parasitic castration, a sum- 

 mary of the chief facts now recounted will be given in order that the reader may have them 

 concisely before his eyes. These facts will then be discussed in their relation to other evi- 

 dence bearing on the problem of sexual differentiation, and finally I append a summary of 

 the conclusions that appear to be suggested by a consideration of the whole matter. 



Summary of Facts. 



1. Of about 5000 specimens of Inachus scorpio examined, all the cases of partial or 

 complete hermaphroditism (in all over a thousand) observed, were found in crabs which carried 

 either Sacculina externa or interna, or else showed unmistakable signs of having been once 

 infected by Sacculina. The only possible exception was a single perfect hermaphrodite, which 

 probably had recovered from Sacculina and undergone several moults so as to lose the scar 

 characteristic of recovered crabs. 



2. About 70# of the males and females of I. scorpio infected with Sacculina showed obvious 

 alterations in their secondary sexual characters: all the infected individuals showed an arrested 

 development of the sexual gland in various degrees, or in certain cases perfect hermaphroditism. 



3. We may therefore conclude that in Inachus scorpio, whenever an hermaphrodite con- 

 dition of any degree is found associated with the presence of Sacculina, the latter has brought 

 about the hermaphroditism of its host. 



4. In Eriphia spinifrons the Sacculina does not appear to have much effect on its host: 

 but a parasitic castration of the male is effected by a species of Entoniscid, which is 

 usually associated with a Sacculina on the same host. 



5. In Pachygrapsus marmoratus parasitic castration is brought about by the Sacculina. The 

 modified hermaphrodite form is of a definite character and is clearly always derived from the male. 



6. In parasitic castration by Sacculina the reduction of the gonad is brought about 

 before the roots of the parasite actually attack that organ, though the final eradication of the 

 gonad, especially in the female, is probably influenced by the irruption of the roots into the 

 germinal tissues. In the case of Entoniscus however the parasite never invades the actual 

 tissues of the gonad. 



7. Infected males assume in various degrees the secondary sexual characters proper to 

 the female: in certain cases of /. scorpio they do this so perfectly that the only sign that 

 they once were males is the presence of a much reduced copulatory style. 



8. The infected females have the swimmerets reduced or rudimentary: they do not 

 however acquire any distinctively male character. 



