248 THE TAPEWORMS 



a true genus but is a collective group of larval tapeworms of the 

 plerocercoid type which cannot be definitely classified because 

 the adult is unknown. 



Echinococcus hydatids. In some parts of the world infection 

 with the hydatids or larvae of Echinococcus is very common, 

 especially in children. In Iceland, where there is very close asso- 

 ciation between the human and the canine population, two or 

 three per cent of the inhabitants are afflicted, and in certain dis- 

 tricts as high as ten per cent. In Australia, also, this tapeworm 

 is common in dogs and its larvae occur in a considerable pro- 

 portion of human beings as well as in stock. In the United 

 States, especially in the southeastern states, it is fairly common. 

 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 

 FIG 94 Eh' to " kiss " them or lick their faces with a tongue 

 coccus granuiosus which, in view of the unclean habits of dogs, 

 may be an efficient means of transmission for 

 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 



