Hog Lung-Worms 273 
always be traced to the presence of two species of lung-worms. If, 
for example, females over 4 cm. were placed in sterile soil, the re- 
sulting larvae were always of two kinds. The first was a slim, active 
form, and the other a rather stout, sluggish form. In this last, ovaries 
were clearly distinguished in the second week of growth. The larvae 
within each group were consistently alike, hence their structural dif- 
ferences could only be due to sex. In soil cultures where no effort 
was made to separate the females on the basis of length, two kinds of 
larvae were also found; namely, the slim, and the active forms. How- 
ever, each of these types could be further divided into two groups, 
which were identical in every respect except size. The presence of 
these larger larvae could be best explained on the basis that they were 
the young of M. brevivaginatus. 
Active larvae were found in scrapings from the sinuses of the 
head, the trachea, and in the mucous discharges from the nose. These 
more active forms were found to be due simply to differences in the 
stage of development. Experiments have shown that the larvae of 
lung-worms remain sluggish for several days after they are hatched, 
during which time important morphological changes occur. 
Attempts to raise the free-living generations were unsuccessful. 
However, when the proper environmental conditions were provided, 
there was little difficulty in growing larvae taken directly from the 
lungs. Such larvae grew actively for a period of four weeks, after 
which time no growth took place but they continued to live indefinitely. 
These results do not agree with the statement made by Dr. von Linden 
and others, that the embryos taken from the lung die almost immediately 
when placed outside the body. It was found that young larvae are very 
susceptible to moisture and temperature changes, to bacteria and de- 
composition products, and to proper aerobic conditions. No difficulty 
was experienced in keeping lung-worm larvae for several days in physi- 
ological salt solution, provided this solution was changed daily, and thor- 
oughly aerated. 
THEORY OF INTERMEDIATE HOSTs. 
In conducting this series of experiments, such animals were used 
as are commonly found in hog runs and wallows, and which might 
readily be a source of infection to the hog. Table 3 shows in tabular 
form the animals with which experiments were conducted and their 
attendant results. 
Most of the flies and lice were caught upon the bodies of dead 
hogs. A count was kept of each species, to find if possible, the per 
cent of infected individuals. Specimens were examined by tearing them 
to pieces in salt solution, and dissecting the contents under a binocular 
microscope. No affected individuals were found. 
Attempts to infect the Ostracoda and Cyclops were made by plac- 
ing them in a beaker of water, into which were also put a large num- 
ber of lung-worm larvae. These were left in contact for ten days, 
during which time daily examinations were made to determine any 
occurrence of parasitism. The results were consistently negative. Ex- 
amination of these crustacea was facilitated by first dissolving out the 
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