ZOOLOGICAL DISTRIBUTION OF DISEASE. 275 
elephantiasis is so frequent that they are manifestations 
of the same disease. 
According to the admirable researches of Dr. Manson 
the chief facts connected with this disease are the 
following :— 
The adult worm is probably taken into the alimentary 
canal of man in drinking-water. From the stomach 
it bores its way into the thoracic duct or some 
lymphatic vessel, and is subsequently joined by one of 
the opposite sex. Here they may live for years, dis- 
charging their embryos into the lymph stream, to become 
distributed by the blood current over the body. 
One of the most remarkable phenomena connected 
with filarial disease is that the embryos disappear from, 
and reappear in, the general blood stream at certain 
periods of the day. Under ordinary conditions the blood 
of an affected individual presents no filaria between the 
hours of nine a.m.and six p.m.; after six p.m. they begin 
to make their appearance and increase in numbers until 
midnight, at which hour as many as 260 filaria have 
been counted in a single drop of blood: from this hour 
they gradually diminish in numbers and at nine o’clock 
in the morning they cannot be detected in the blood. 
During the night, and whilst the filarial migration is at 
its height, mosquitoes visit the body, and Manson has 
identified the female, a particular variety of Cw/ex which, 
by the structure of its oral appendage, abstracts blood 
from the filariated individual, be it man or beast. As 
the culex selects eight o’clock in the evening as the feed- 
ing hour it necessarily follows that the embryo filariz 
are taken into the stomach of the mosquito, and Manson 
