F. A. Potts 
45 
of toxin-secretion by parasites and its effect upon the blood of the 
host. This is an example of the important conclusions which are to 
be reached from investigation of parasitism in the Yertebrata, and it 
may be suggested that observation of the reproductive organs in acute 
cases of infection like filariasis and ankylostomiasis might prove of 
great interest 1 . It is perhaps fair to conclude that a marked change 
in the reproductive glands themselves would have been chronicled in 
clinical observations had it existed, but we may expect perhaps to 
find minor changes in the sexual economy or the secondary characters, 
traceable to the influence of a specific change wrought by the parasite. 
A preliminary notion of the nature and extent of the changes in 
the genital system due to parasitism has been given and it is now 
proposed to bring forward the evidence from later research which 
makes it possible to form a much clearer idea of the phenomenon. 
Geoffrey Smith in 1906 published a very full account of the effect 
of the association of Sacculina with the spider crab ( Inachus ) embodying 
researches extending over three years. Before attempting to summarise 
this remarkable work, a little definite information about the life-history 
of the parasite and the sexual differences of the host must be given. 
The larvae of Sacculina are free-swimming and are not transformed 
directly into the individual we know as the adult, but pass through 
a wholly internal existence in the body cavity of a crab, where they 
rapidly absorb nourishment from the blood-serum. A definite position 
in the abdomen of the host is attained, a closely ramifying system of 
absorptive roots is developed and the time approaches when the 
parasite becomes partly external. This change in the even course 
of its life can only take place through the agency of a moult. In 
the new cuticle which forms under the old carapace during moulting, 
modifications are found which concern the secondary sexual characters 
and these, it must of course be concluded, are due to the influence of 
the embryonic though maturing Sacculina. The infected crab never 
moults again while its parasite lives, indeed such a moult would be 
fatal to the parasite, and so while the gonads are subject to the 
constant action of the Sacculina, the external sex characters only 
preserve for us the early effects of the association. It is impossible 
certainly in the present case to observe whether the changes become 
1 Turner (1908) mentions the widespread belief in Mozambique that impotence and 
sterility accompany bilharziosis. As an interesting sidelight on the relations of parasite 
and host we may quote his statement of the frequent disappearance of the parasites 
at the onset of puberty. 
