Hog Lung-Worms 275 
It would appear from these experiments that M. apri and M. 
brevivaginatus have no intermediate hosts, but that active, growing lar- 
vae may occasionally invade the lungs of another animal than the hog. 
In such a case, however, they are unable to continue their development. 
An exception might be cited in the case of the human host; for several 
cases are on record where worms, seemingly identical with those of the 
hog, have been found in human lungs. 
THEORY OF PARTIAL DEVELOPMENT IN THE SOIL. 
Analyzing the facts thus far considered, it would appear that these 
parasites spend a part of their development within the soil. To grow 
these worms, small wooden boxes were used parafined on both surfaces. 
Different kinds of soils were found to give different results. The op- 
timum soil types were, first, a humus to which some sand had been 
added, and second, soil scrapings from the bottom of an old manure 
pile. Neutral or slightly alkaline soils gave the best results. It ap- 
pears that young larvae feed within the soil, for they made no appre- 
ciable growth in the sterile plots until such time as seeds were planted 
and had begun to sprout. In this regard oats and beans gave satis- 
factory results, and they served moreover, as excellent indices in the 
control of moisture. Check experiments were in all cases conducted with 
soil sterilized under a pressure of fifteen pounds of steam for one hour. 
Besides the soil types mentioned above, moisture and temperature 
were found to be factors of prime importance. The temperatures that 
gave the best results and at which the larvae were the most active, 
ranged from 35 to 40 degrees Centigrade, and preferably within the 
upper limits of this range. The moisture requirement is peculiar and 
must be very exact. As these parasites require an abundance of oxy- 
gen, all experiments involving them had to take this factor into con- 
sideration. Too much and too little moisture were both detrimental. 
The former excluded the air from the soil, and the latter condition in- 
hibited motion and proper metabolic development. The larvae grew most 
rapidly in soils that were damp and porous, and that contained much 
organic matter. Moisture requirements were best controlled by means 
of vegetation growing within the plots. Newly hatched larvae are 
especially succeptible to these changes, and they are readily killed if 
the soil is allowed to dry out. 
In their development these larvae showed many marked pecu- 
liarities. Under ideal conditions they increased rapidly in size during 
a period of four weeks. During this time the digestive tract and sex 
organs become well defined. The bodies of both sexes terminate in a 
spine-like process, which is much longer in the males. These last are 
also more slender and active than the females. Motion is effected by 
side to side contractions of the body, which cause the worm to move 
rapidly along especially in a media of semi-fluid consistency. Plate I 
shows the appearance of these larvae during different stages of growth. 
A decided metabolic difference exists between partly developed 
larvae and those which are just hatched. Experiments conducted with 
different digestive ferments, such as bile, saliva, liver extract, etc., 
showed that the former larvae are much more resistant to the action of 
