176 
THE WORM GROUP 
in stagnant water and in streams. It is frequently brought 
into the laboratory and can easily be kept alive in aquaria. 
The liver fluke is a parasitic flatworm which each year 
causes the death of many sheep by injuring their livers. 1 
Like some other parasitic animals the liver fluke requires 
two hosts to complete its development. The hosts of the 
fluke are the sheep and certain snails. The adult liver 
flukes form eggs and sperms in the liver of the sheep. The 
fertilized eggs partially develop in the sheep; then as 
embryos they pass down the bile duct 
into the intestine and then out of the 
body. 
The ciliated (sil'i-a-ted) larva then 
makes its way into water or along dew- 
covered grass. If it comes in contact 
with a water snail in the water or a land 
snail on the grass, it enters the body of 
its second host, otherwise it dies. Once 
inside the body of the snail it completes 
a complicated development. By a 
bud-like process many young flukes are 
formed which finally emerge from the 
snail and make their way to the grass 
stems on which they encyst themselves. 
If this grass is eaten by a sheep, the diges¬ 
tive fluids set free the young fluke, which 
goes up the bile ducts to the liver, where it grows to maturity. 
160. Trichina. — Another unsegmented worm that is of 
economic importance is the Trichina (tri-ki'na), now gen¬ 
erally called Trichinella (tri'kl-nel'la). This worm lives 
in the intestine of mammals and from the intestine mi¬ 
grates into the muscles of its host. In the muscle it becomes 
encysted and remains until the flesh is eaten by some other 
1 The Animal Parasites of Sheep. Dr. Cooper Curtice. Bureau Animal 
Industry, United States Department of Agriculture, 1890. 
Figure 164.— Trichi¬ 
nella. 
A picture of this para¬ 
sitic worm in the mus¬ 
cles of man. Note the 
membranous sac which 
encloses it. This is the 
form of the sac after the 
worm comes to rest in 
the muscles. The worm 
is then said to be en¬ 
cysted. 
