220 



THE ANIMAL PARASITES OF MAN 



in fact, the cercarice (which in this case do not acquire an oar-like tail) 

 do not swarm out of, but encyst themselves within their sporocysts. 

 The development, moreover, may be further complicated by rediae 

 appearing in addition to the sporocysts, though this occurs in the 

 first intermediate host and not in a second one. 



Animals that harbour adult digenetic Trematodes thus become 

 infected by ingesting encysted cercariae, which either occur (i) in 

 certain animals (second intermediate hosts) on which they feed, or 

 (2) in water, or (3) on plants, or finally (4) in the first intermediate 

 host ; whereas animals harbouring encysted cercariae have been directly 

 infected by the corresponding tailed stage, and animals harbouring 

 germinal tubes (sporocysts or rediae) have been infected by the 

 miracidia. 



Thus certain species of ducks and geese become infected with 

 Echinostoma e.cliinatum by devouring certain water-snails (Limnans, 



a be 



FlG. 131. Development of Fasciola hepatica, L. a, the miiacidium in optical section 

 showing cephalic lobe, X-shaped eye-spot resting on the cerebral ganglion, two germ balls ; 

 below each of these a flame cell, and still lower germ cells lying in a cavity (primitive body 

 cavity). l>, young sporocyst with two eye-spots, and germ balls ; the cells lining the cavity 

 are not shown, f, older sporocyst with a young redia. Magnified. (After Leuckart.) 



Paludina) in which the encysted cercariae occur. Oxen become infected 

 with Paramphistomum cervi (= Amphistomum conicnm) by swallowing 

 with water, cysts of this species which occur at the bottom of puddles 

 and pits. Sheep are infected with Fasciola hepatica by eating grass 

 to which the encysted cercariae of the liver-fluke are attached ; our 

 song-birds infect themselves or their young with Urogonimus macro- 

 stomus by tearing off pieces containing the corresponding sporocysts 

 which are full of encysted cercariae from snails (Succinea amphibia'), 

 which act as the first intermediate hosts, and eating, or offering their 

 young these pieces. 



(i) The MIRACIDIA of the digenetic Trematodes are compara- 

 tively highly organized, and the mode of their formation from the 



