inches in length and about as thick as an
ordinary pin, may be seen, if present in any considerable number, actively
wriggling about like little snakes.

Cattle become infected with these parasites by grazing on pastures on which
infested cattle, sheep, or goats have grazed and scattered their droppings.
The worms in the stomach produce a multitude of eggs (fig. 14_c_) of
microscopic size, which pass out of the body in the feces. In warm weather
these eggs hatch in a few hours. If the temperature is below 40 deg. F., they
remain dormant, and if below freezing, they soon die. The eggs are also
killed by dryness; moisture, on the other hand, favoring their development.
The larvae which hatch from the eggs are microscopic in size, and, like the
eggs, are, at first and until they have developed to a certain stage, very
susceptible to freezing and drying. In very warm weather the larvae complete
their development, so far as they are able to develop outside the body, in
two or three days. In cooler weather the time required for this development
is longer, and at temperatures below 70 deg. F. 10 days to several weeks may be
necessary. The larvae are then ready to be taken into the body. The eggs and
early stages of the larvae apparently do not develop if swallowed, and only
the completed larval stage seems to be infectious. In this stage the larvae
migrate up grass stalks (fig. 15) or other objects, showing activity
whenever the air is saturated with moisture; that is, during rains, fogs,
and dews. When the air becomes dry and the moisture evaporates from the
grass the young worms cease their activity, resuming their migrations when
the air again becomes overladen with moisture. Larvae which have developed
to the infectious stage, unlike the eggs and early larval stages, are able
to survive long periods of freezing and dryness. In two weeks to a month
after the embryos are swallowed they reach maturity and begin producing
eggs.

_Preventive treatment._--Preventive measures are important. As moisture
favors the development of the embryos, high sloping ground is preferable
for pastures. If low ground is used, it should be properly drained. The
pasture should not be overstocked. Burning over the pasture will destroy
most of the young worms on the grass and on the ground, and this means of
disinfection under certain circumstances may be very advantageously used.
The herd should be changed to fresh pasture as often as possible. Catt