72 The effect of the Parasites on their Hosts. 



they are at the same time of the typically adult female nature, and not a return to any 

 embryonic or ancestral condition. 



To sum up the effect on the male organization of Inachus scorpio it is seen, that in ali 

 the secondary sexual characters the infected males become in various degrees typi- 

 cally female. In extreme cases where a male has assumed the chelae, abdomen and abdominal 

 appendages of a female we can only be sure that it is a male by the presence of a copulatory 

 style and by the slightly smaller width of the abdomen in comparison with a typical female. 



B. Effect on the primary sexual character. 



By dissecting a large number of crabs (about 500 infected and 400 uninfected) it has 

 been possible to establish that some effect is always exerted by the parasite on the gonad. 

 But just as in the case of the secondary sexual characters, the degree of affection varies 

 greatly; and to a certain extent, there is a correlation in one and the same animai between 

 the degree of degredation of the gonad and of the secondary sexual characters. That this 

 correlation is not exact would be expected from the fact that the atrophy of the gonads con- 

 tinues after the Sacculina has become external, whereas the secondary sexual characters do 

 not change after this point. The effect on the gonad is most clearly perceived during the 

 breeding-season when the females instead of having red ovaries have small white ovaries which 

 may be either much shrunken or else in a state of actual disintegration ; while the testes and 

 vesiculae seminales of infected males are always obviously greatly reduced in comparison 

 with those of uninfected individuals. In many cases the ovaries and testes with their ducts 

 may be altogether absent, and this is always associated with very highly modified secondary 

 sexual characters. At the opposite extreme we may meet with an occasionai female which 

 has actually shed its ova, which are then carried under the abdomen with the parasite, but 

 these eggs are always few, and dissection of such a female has invariably revealed that its 

 gonads internally are much retarded in development. 



As to how the parasite brings about this reduction and in many cases final atrophy of the 

 gonad there is a distinction between males and females. In the females the roots of the 

 parasite may actually penetrate the epithelium of the ovary and ramify among the degenerating 

 ova, but in the males the roots never appear to penetrate the testicular tubes. I have also 

 good evidence to show that the actual irruption of roots into the ovary is often only a second 

 phase in the atrophy of the gland, and that a reducing effect appears before the roots have 

 actually penetrated into its substance. It must be concluded that the Sacculina primarily causes 

 the reduction of the gonad by interfering with the general metabolism of the body, possibly 

 by depositing in the body some fermentative or excretory substance, or more probably by 

 altering its general state of nutrition. This disturbance of the general metabolism must inter- 

 fere in some manner with the sexual condition of the body, but with this subject we will 

 deal more fully at the end of the chapter. 



