100 CRUSTACEA CHAP. 
to possess a root-system, but is attached to its host by a tube 
which passes right through the mesentery and opens into the 
mantle-cavity of the parasite. It may be suggested that this 
tube corresponds to the stalk of the normal Cirripede, but its 
exact mode of formation would certainly throw much light on 
the question of Rhizocephalan development. 
Phenomena of Growth and Sex in the Crustacea. 
In the foregoing account of the Cirripedia we have met with 
certain peculiar sexual relations in which closely allied species 
exhibit marked differences in regard to the distribution of the 
qualities of sex among their individuals; we have seen that the 
majority of species are hermaphrodite, unlike most Crustacea 
which, with the other exception of the parasitic Isopoda, are 
normally dioecious; and that in some species complemental 
males exist side by side with the hermaphrodites, while, in yet 
others, the individuals are either females or dwarf males. 
Before examining the causes of these conditions, it will be 
opportune to consider a number of facts which throw light on 
the question of sex and hermaphroditism in general. We may 
then return to the discussion of the hermaphroditism found in 
particular in the Cirripedia and Isopoda. 
Parasitic Castration.—Giard' was the first to observe that 
a number of parasites exert a remarkable influence on the sexual 
characters of their host, such that the generative glands become 
reduced, or may completely degenerate, while the secondary 
sexual characters become materially altered. This was proved 
to occur in the most widely different hosts, affected by the 
most widely different parasites (e.g. Crustacea, Insecta, Worms). 
Moreover, it was apparent that the affection does not consist 
in the parasite merely destroying the generative organs, 
with which it often does not come into contact, but rather in the 
general disturbance of the metabolism set up by its presence. 
The most completely studied cases of parasitic castration are 
those of the Rhizocephalous Sacculina neglecta, parasitic on the 
spider-erab, Lnachus mauritanicus, and of Peltogaster curpatus 
1 Bull. Sc. Dép. Nord (2), 10 Ann. xviii., 1887, p. 1. Ibid. (3), i., 1888, p. 12; 
and other papers. 
2 G. Smith, Zoc. cit. chap. v. LZ. scorpio should be J. mauritanicus throughout 
this Monograph. 
