The Taenia Echinococcus in its adult form is found in the intestinal tract of the dog, the larval form occurring in man and some of the lower animals.

It is quite short, about 4 or 5 cm. long, and consists of four segments. The head, which forms the first, is long, has four suckers and a rostellum having from 28-50 hooklets arranged in a double row.

The fourth segment is the largest and constitutes about two-thirds of the entire worm. It alone possesses a uterus which consists of a median portion with a few lateral branches. It contains numerous long oval eggs with very thin shells. The eggs enter the intestine, the shells are dissolved and the embryos set free. They penetrate various tissues, particularly the liver, become encapsulated, and slowly develop into cysts whose walls are made up of two layers - the outer cuticular, formed from the connective tissue of the host, and the inner granulocellular layer.

In the course of some weeks small projections grow from the inner layer and project into the primary cyst. These buds have hooklets and suckers and are embryo parasites. This variety of cyst is the Echinococcus scolecipariens.

In the Echinococcus hydatidosus or E. endogenes, daughter and even grand-daughter cysts develop inside the original cyst. They probably result from a cystic change in the buds already mentioned. Sometimes secondary cysts form on the outside of the wall, the E. exogenes.