The dog is the host for _Taeniae marginala_, _coenurus_ and _echinococcus_.
The larvae forms of these _taeniae_ are the _Cysticercus tennicollis_,
_Coenurus cerebralis_ and _Echinococcus polymorphus_. _C. tennicollis_ is a
parasite of the serous or lining membranes of the body cavities. It is not
of great economic importance. _C. cerebralis_ is a parasite of the brain of
sheep, and may cause a heavy death rate in flocks that are infested with
it. _E. polymorphus_ is a parasite of the liver, but it may occur in other
organs.

THE THORN-HEADED WORM OR ACANTHOCEPHALE.--This parasite requires a
secondary host. In this case a particular species of the May-beetle larva
or white grub that is commonly found about manure piles and in clover
pastures is the host. The hog eats a white grub that is host for the larval
form. The digestive juices free the larva, it then becomes attached to the
intestinal mucous membrane and develops into the adult thorn-headed worm
(Fig. 74). This parasite is characterized by a hooked proboscis or thorn at
its anterior extremity, and the absence of a distinct digestive tract. The
male is much smaller than the female. The eggs are passed out of the
intestine with the faeces.

[Illustration: FIG. 74.--Thorn-headed worms.]

THE ROUND-WORMS OR NEMATOIDS.--Round-worms are very common parasites of
domestic animals (Fig. 75). This group of worms is characterized by their
cylindrical form, the presence of a true digestive canal and the separation
into two sexes, male and female. The life history is more simple than in
the flat worms. Intermediate hosts are not required for the development of
the common forms. The eggs and embryos are deposited by the female in the
intestinal tract, air passages, or excretory ducts of the kidneys of the
host. Development may be completed here, or the eggs and embryos are passed