of the parasite was _Inachus scorpio_ (or _dorsettensis_). Figures showing
the changes in the abdomen produced by the presence of _Sacculina_ are
given in Doncaster's _Determination of Sex_, Pl. xv. _Sacculina_ is one of
the Cirripedia, and therefore allied to the Barnacles. It penetrates into
the crab in its larval stage, and passes entirely into the crab's body,
where it develops a system of branching root-like processes. When mature
the body of the _Sacculina_ containing its generative organs forms a
projection at the base of the abdomen of the crab on its ventral surface,
and after this is formed the crab does not moult. Crabs so affected do not
show the usual somatic sexual characters, and at one time it was supposed
that only females were attacked. It is now known that both sexes of the
host may be infected by the parasite, but the presence of the latter
causes suppression of the somatic sex-differences. The entry of the
parasite is effected when the crab is young and small, before the somatic
sex-characters are fully developed. The gonads are not actually
penetrated, at least in some cases, by the fibrous processes of the
parasite, but nevertheless they are atrophied and almost disappear. In
_Inachus_ the abdomen of the normal male is very narrow and has no
appendages except two pairs of copulatory styles. The abdomen of the
female is very broad, and has four pairs of biramous appendages covered
with hairs, the normal function of which is to carry the eggs. The effect
of the parasite in the male is that the abdomen is broader, the copulatory
styles reduced, and biramous hairy appendages are developed similar to
those of the female, but smaller. In the female the abdomen remains broad,
but the appendages are much smaller than in the normal female, about equal
in size to those of the 'sacculinised' male. Smith interpreted the
alteration in the male as a development of female secondary characters,
but it is obvious from the condition in Macrura or tailed Decapods, like
the lobster or crayfish, that the abdomen or tail of the male originally
carried appendages similar to those of the female, and that the male