248 THE TAPEWORMS 



The adult of Echinococcus (Fig. 94) is a minute tapeworm 

 found in dogs and sometimes in other carnivorous animals. It 

 measures only from one-tenth to one-fifth of an inch in length. 

 The mature worm consists of a tiny scolex with four suckers and 

 a double crown of hooks, followed by an unsegmented neck and 

 three gradually larger proglottids, the ultimate 

 one of which is larger than all the rest of the 

 worm and contains about 500 eggs in the uterus. 

 Echinococcus may occur in hundreds or even 

 thousands in the intestine of dogs, though it 

 often escapes notice on account of its. small size. 

 The eggs of the worm, dropped in pastures 

 with the faeces of infected dogs, ordinarily de- 

 velop in sheep, cattle or other herbivorous 

 animals. Human infection usually results from 

 too intimate association with dogs, and children 

 especially are liable to infection by allowing dogs 

 to " kiss " them or lick their faces with a tongue 



FIG. 94. Echino- 



coccus yranuiosus^ which, m view of the unclean habits of dogs, 

 from dog. x io. mav b e an efficient means of transmission for 



(After Leuckart.) 



the tapeworm eggs. 



The hydatids develop in many different organs of the body. 

 The liver is the favorite site, after which, in order of frequency, 

 come the lungs, kidneys, spleen, intestinal walls, membranes 

 lining the body cavity, heart, brain and various muscles. Some- 

 times a single host is invaded by the hydatids in several different 

 organs. / 



The development of the embryos is very slow indeed. In a 

 month after reaching their destination in the liver or other organs 

 they are in the form of little globular bodies, enclosed by a 

 capsule produced at the expense of the host. A cyst measures 

 about one mm. (3^ of an inch) in diameter. By the end of the 

 fifth month it has grown to the size of a walnut. The membrane 

 of the bladderworm itself is very delicate, but the capsule formed 

 by the host is thick and tough. The enclosed fluid is transparent 

 and nearly colorless, and is composed of various materials which 

 have permeated in from the blood and tissues of the host, and 

 of the waste products produced by the growth of the parasite. 



When the hydatid has reached this stage in its development 

 (Fig. 95) there grow into its cavity from the inner surface little 



