2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



Of the four worm groups named, the cestodes are ahnost exchi- 

 sively heteroxenous. We have the rare exception of Hymenolepis 

 nana of the rat, which develops as an adult in the small intestine of 

 the rat, produces eggs which pass out in the feces and by contamina- 

 tion of the rat's food infects the rat with the larval stage of the 

 tapeworm, a small cysticercoid which develops in an intestinal villus 

 of the rat, and which then returns to the lumen of the intestine to 

 become an adult worm, the rat serving as both the primary and the 

 intermediate host for the worm. Even in this case it has been claimed 

 that rat fleas may act as intermediate hosts, but this has not yet been 

 confirmed. This may be one of those cases in which a parasite can 

 use an intermediate host or do without it. We seem to have similar 

 cases in such parasites as the common gape-worm of poultry which 

 can utilize the earthworm as an intermediate host or can infect 

 chickens directly, and the blackhead organism which can use the 

 cecum worm as an intermediate host or can infect turkeys directly. 

 In the great majority of cases, the tapeworm is adult in an animal 

 which eats the intermediate host animal and thereby becomes infested 

 with the adult worm as the larval worm from the intermediate host 

 comes to maturity in the primary host. In some of the bothriocepha- 

 lids, in cases in which the life histories are well known, the eggs of 

 the adult tapeworms present in the primary host, a higher vertebrate, 

 hatch on entering water, infect such small animals as the copepods, 

 and develop in the body cavity of these first intermediate hosts to an 

 early larval stage, the procercoid. When such infested entomostracans 

 are eaten by such intennediate hosts as fish, the procercoid undergoes 

 further development and becomes a plerocercoid in the flesh of the 

 fish. When infested fish are eaten by a suitable higher vertebrate, such 

 as a human being or dog, the plerocercoid develops to the adult tape- 

 worm in the small intestine of this host. 



Among the flukes we have one large group, the Monogenea, which 

 are usually ectoparasitic, mostly on fish, but sometimes endoparasitic, 

 as in the respiratory tract of turtles or the urinary bladder of amphib- 

 ians, and these flukes are monoxenous, developing without an inter- 

 mediate host ; another large group, the Digenea, are regularly 

 endoparasitic and are heteroxenous. The digenetic flukes occurring in 

 vertebrates produce eggs which pass out in the feces or urine and 

 hatch after entering water. Usually the newly hatched worm (miracid- 

 ium) attacks a mollusk host and develops in this host to the stage 

 known as a cercaria. It may now be eaten by its primary host, or may 

 escape and encyst in water or on vegetation and be swallowed by its 

 primary host, developing in either case to an adult worm, or it may 



