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 off with the body excretions. They may live for a short time outside the animal body, or undergo certain development and again infest a host of the same species from which they came, through the water, grass and fodder that the animal may take into its digestive tract. [Illustration: FIG. 75.--Large round-worm in intestine of hog.] The following species of nematoids are common parasites of domestic animals: SOLIPEDS Species Organ Ascaris megalocephala Intestines Sclerostoma equinum Large intestine and blood-vessels Sclerostoma tetracanthum Large intestine Oxyrus curvula Large intestine Oxyrus mastigodes Large intestine CATTLE Species Organ Strongylus convolutus Abomasum Ascaris vituli Small intestine (calves) Strongylus ventricosus Small intestine Oesophagostomum inflatum Large intestine Trichocephalus affins Large intestine Strongylus micrurus Bronchi Strongylus pulmonaris Bronchi SHEEP