LIFE HISTORY OF ASCARIS LUMBRICOIDES. 19 
anterior end of body 0.12 mm., distance of anus from tip of tail 
0.08 mm., and distance of genital primordium from tip of tail 0.48 
mm. The 
body is of 
nearly 
uniform 
diameter 
through- 
out, attenuated slightly from the base of the esophagus 
forward, and gradually from the beginning of the 
posterior third of the body backward, diminishing to 
about half its maximum diameter in the anal region; 
tail conical. Along each lateral line is 2 well-marked 
membrane (shown in cross section in figure 6). The 
mouth is small, pharynx very short, the esophagus 
beginning almost immediately back of the mouth; lips 
not conspicuous, the knobhke process on the anterior 
aspect of the head characterizing 
the newly hatched larva being no 
longer present. Just in front of 
the bulbous posterior end of the 
esophagus, sometimes on _ the 
right side, sometimes on the left 
side, is the large nucleus of a 
cervical gland. The genital pri- 
mordium is small, not more than 
0.015 mm. in diameter, oval in 
shape, consisting of a very few ~ 
cells, situated on the ventral side 
of the intestine some distance 
posterior of the middle of the 
body. 
In the living worm the eso- 
phageal region is clear and trans- 
parent, the intestinal region yel- 
lowish-brown through the pres- 
ence of numerous granules of 
this color in the intestinal cells 
(figure 4). 
Fie. 3.—Ascaris suum. Larva from lung of 
rabbit 10 days after infection. Slightly 
flattened by pressure of -cover glass. 
Magnified 150 times. 
RELATION OF HOST TO SIZE OF DEVELOPING LARVA. 
Parasites that are able to live in more than one species of host 
animal frequently exhibit differences of size in different species of 
animals. A good example is the gapeworm (Syngamus trachealis), 
