Experiments in Rhythmetic Periodicity 149 
most strongly marked in the deserts. There is not always a 
Sharp limit between day and _ night communities; in polar 
regions there is no night fauna, and in the tropics the latter is 
very rich. 
Such night and day changes are found not only in free living 
animals, but also exist among parasites of mammals and birds. 
Owing to the fact that most mammals sleep regularly either by 
day or by night, there exist corresponding rhythmical changes 
inside their bodies, especially in temperature. In both birds and 
mammals, the body is slightly colder during sleep. This rhythm 
depends entirely upon the activity of the animal, since nocturnal 
birds like owls have the normal rhythm reversed (i. e., they are 
warmer at night), and this in turn can be reversed by changing 
the conditions under which they live so as to cause the birds to 
come out by day and sleep by night. There are certain nematodes 
parasitic in man which show the effects of sleep rhythm in a 
very remarkable way. The first species (Filaria bancroftt) lives 
as an adult in the lymphatic glands of man in tropical countries, 
but its larvae live in the blood. In the daytime these larvae 
retire to the inner parts of the body, mostly to the lungs, but 
at night they issue forth into the peripheral circulation, appear- 
ing first about 5 to 7 in the evening, reaching a maximum about 
midnight and disappearing again at about 7 or 8 o’clock in the 
morning. This reaction can be reversed if a person stays up all 
night and sleeps in the day, which shows that the round 
worms’ activity is affected by the rhythmical changes in the 
conditions of the body like those described above. Another 
Species of Filaria (loaloa) has larvae which live in the blood of 
man, but unlike the other species, these larvae come out only 
In the day, disappearing at night. It is stated that this perio- 
dicity is not affected by reversal of sleep, but presumably it 
must originally have been caused by some rhythm in bodily 
environment. Manson-Bahrn* mentions a third species which 
has larvae in the blood, which occur in the peripheral circulation 
equally by day or by night. 
The habits of these worms have a very important bearing 
upon the means of transmission from one man to another; 
F. bancrofti is transmitted by blood-sucking mosquitoes which 
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*Manson’s Tropical Diseases. 
