STUDIES IN THE EXPERIMENTAL ANALYSIS OE SEX. 593 
quently infected with a species of rhizocephalous Cirripede 
called Sacculina neglecta (5). This parasite at first lives 
a free existence as a minute larva; it then fixes itself to a hair 
on the outside of its host and passes into the body of the 
latter a small gi'oup of cells which find their way to the blood- 
space round the intestine. Here they begin to gi'ow very 
rapidly into a branched tumoui--like body which sends its 
ramifications into every part of the body-cavity of the crab. 
A certain part of the tumour becomes applied to the ventral 
body-wall of the crab at the junction of thorax and abdomen, 
and at this point the reproductive organs, etc., of the adult 
Sacculina ai’e developed and finally thrust to the outside in 
a muscular bag which remains attached to the crab and 
swells to a large size, gaining its nutriment from the system 
of branching roots which continue to multiply and grow 
inside the crab’s body. 
Now the chief effect which the parasite exerts on the crab 
is to cause the complete or partial atrophy of the internal 
generative organs, with their ducts, while remarkable changes 
take place in the structure of the external secondary sexual 
characters. Of 1000 specimens of Inachus infected with 
Sacculina examined by me at Naples, 70 per cent, of both 
males and females showed very distinct alteration in their 
secondary sexual characters, while all showed some degree of 
reduction or atrophy of the gonad. Of the many thousands, 
at present well over 5000 specimens, of uninfected Inachus 
examined, only one specimen showed any trace of the changes 
such as were inet with in the infected individuals. This speci- 
men, which was a perfect hermaphrodite both externally and 
internally, may have been an instance, such as occurs with 
extreme rarity in decapod Crustacea, of hermaphroditism 
apart from parasitic castration, but it is equally possible that 
it was really a crab that had recovered from an infection with 
Sacculina, and had undergone several moults so as to lose 
the scar characteristic of crabs that have been once infected. 
I mention these facts with regard to the numbers of specimens 
examined, because it is important to realise not only the 
