10 



BULLETIN 260, U. S. DEPARTMENT OF AGRICULTUBE. 



may attain a length of 2 or 3 feet. In the terminal segments are 

 eggs, and these segments with the contained eggs pass out in the 

 feces of the dog and contaminate vegetation, soil, and water. Such 

 herbivorous animals as sheep, which graze over range or pasture 

 contaminated in this way, pick up these eggs as they feed and swal- 

 low them. In the stomach of the sheep the shell is digested and 

 the small, hooked embryo released. The embryo bores its way 

 through the wall of the digestive tract and into the blood vessels 



and is carried around until it lodges 

 somewhere. Embryos which do not 

 lodge in the central nervous system 

 start to grow, but very soon perish. 

 Very commonly, however, the para- 

 site makes its way to the central 

 nervous system, lodging as a rule in 

 the brain, though it occasionally oc- 

 curs in the spinal cord. In the brain 

 the embryo grows to form the blad- 

 der worm or ccenurus, and this may 

 attain the size of an egg, or even a 

 larger size. As it grows it presses 

 upon the adjacent portion of the 

 brain and destroys it. The pressure 

 and the irritation, due to the hooks 

 with which the tapeworm heads of 

 the coenurus are provided, cause verj' 

 distinctive symptoms, the sheep com- 

 monly holding its head in an odd 

 position and walking in a circle 

 toward one side or the other. Un- 

 less the ccenurus is removed by oper- 

 ation the sheep invariably dies. 

 When the brain of such a sheep is 

 eaten by dogs — and dogs very read- 

 ily eat the brains of sheep by licking 

 them out through the large opening 

 at the base of the skull — ^the ccenurus is ingested with the brains 

 and the tapeworm heads pass to the intestines of the dog and give 

 rise to the adult tapeworms. As in the case of the hydatid, the gid 

 parasite must always be transmitted from the dog to other animals 

 which eat the eggs from the dog tapeworm, and from the other ani- 

 mals to the dog by the dog eating the brain or at least the coenurus 

 from the brain of a giddy animal. 



Fig. 5. — ^Adult gid tapeworm from the 

 dog. Natural size. (Specimen No. 

 4031, Bureau of Animal Industry 

 helmlntliological collection.) 



