242 THE TAPEWORMS 



They have been reported in a considerable number of other 

 animals also but the cases are very doubtful. The fact that the 

 larval stage can develop in man makes the species particularly 

 dangerous on account of possibihty of self-infection, either by 

 contaminated hands or by a reversal of the peristaltic movements 

 of the intestine which throws the ripe proglottids of the worm back 

 into the stomach where the embryos in the eggs are liberated by 

 the gastric juices. This is discussed further on p. 251. 



The Dwarf Tapeworms. — The dwarf tapeworm, Hymenolepis 

 nana (Fig. 90A), is the smallest tapeworm found in man, but 



it often occurs in such numbers as to 

 cause much irritation in the intestine. 

 It is a common parasite in Italy, and 

 occurs throughout the warm parts of 

 Europe, Asia, Africa and America. It 

 is probably much more common in the 

 United States than is generally sup- 

 posed, since it can easily be overlooked 

 unless the faeces are microscopically 

 examined for eggs. It is probably a 

 common parasite of rats and mice as 

 well as of man, though the rodent par- 

 F^ 90. X dwarf tapeworm, ^sitc is bdieved by some parasitolo- 

 Hymenoiepis nana, X 7 (after gists to be a distinct species, H. mttnna. 



Skfis?:^)':' ''•""""' ^'™ stiles considers the rodent parasite a 



sub-species, H. nana fraterna. 



The adult worm, which consists of from 100 to 200 proglottids, 

 is usually little over an inch in length and less than one mm. (^V 

 of an inch) in width. The scolex (Fig. 87G) has four tiny suckers 

 and a crown of little hooks. The ripe proglottids (Fig. 84J) 

 differ from those of the large tapeworms in being much wider 

 than long, with the enlarged uterus in the form of a solid mass, 

 partially divided into compartments instead of being branched. 



As regards life history, the dwarf tapeworm is commonly be- 

 lieved to pass both its larval and adult stages in a single host, 

 contrary to what occurs, so far as is known, in any other tape- 

 worm. The eggs (Fig. 90B), eaten by a rat or man, liberate six- 

 hooked embryos in the small intestine, where they enter the villi 

 and transform into cysticercoid bladderworms (Fig. 86B), which 

 in turn fall into the cavity of the intestine, attach themselves 



