164 
Development of Hook-worms 
conditions, i.e. where there is a sufficiency of moisture and air and not 
too great heat. In the laboratory they have been kept alive for over 
18 months in plain water at a temperature of about 60° F. It can hardly 
be doubted that they will live fully as long under natural conditions, 
unless it be that they are attacked and devoured by other animals 
such as aquatic insects. On this matter, however, we have no informa¬ 
tion. 
After this stage no further progress in the development of the larvae 
can take place until the advent of a suitable host and a suitable 
opportunity for entry into that host. The most usual mode of entry 
seems undoubtedly to be by penetration of the skin, the commonest 
route is via the bare feet, but the hands and indeed any area of the 
body may be invaded. On the other hand infection may be acquired 
through the mouth by drinking water containing the larvae. At the 
present time it is generally considered that infection through the bare 
feet is by far the commonest method. 
It is unnecessary here to enter into details of the controversy regarding 
skin infection. Suffice it to say that the discovery first made by Looss 
in 1898 has been confirmed by a large number of independent observers, 
and that the ranks of those who have opposed contradictory beliefs 
have been greatly thinned. 
It is also unnecessary to deal here with the further remarkable 
life-history of the worm. All that need be said is that after penetrating 
the skin the larva bores its way into a lymphatic or blood vessel and 
is carried in the blood streani to the lungs. Here it penetrates into 
the air chambers and makes its way along the bronchial tubes into the 
trachea. Thence it passes over into the oesophagus and so down into 
the stomach and intestine where it completes its life cycle. 
Life-History under Abnormal Conditions. 
The matter of the foregoing section has had reference almost entirely 
to the life-history and development of the hook-worms under normal 
and natural conditions. A considerable number of disturbing factors 
may be introduced. As already mentioned the three chief factors 
concerned in development are air, temperature and moisture. The 
effects of these can, in most cases, be very readily estimated. There 
are, however, several other factors to be reckoned with, the effect of 
which cannot be so easily determined. The first of these is the con¬ 
dition of the eggs themselves on being passed. 
