142 THE ANIMAL PARASITES OF MAN 



larval stages of the forms that will ultimately become adult 

 (cercaria) are produced, or the cercaria are only brought forth after 

 having gone through another intermediate generation, that of 

 the REDIAE (fig. 78), which are always provided with an intestine. 

 The Cercaria, as a rule, leave their host and move about in the water 

 with the assistance of a rudder-like tail (fig. 76). After a little 

 time, however, they usually again invade an aquatic animal (worms, 

 molluscs, arthropoda, fishes, amphibians), then they lose their tail 

 and become encysted ; here they wait until they attain the terminal 

 host suitable for them, in company with their bearer, and in this 

 new situation they settle down and reach maturity. Or, again, 

 the cercaria may encyst themselves in the water or on foreign 

 bodies (plants) and wait until they are taken up direct by the 

 terminal host [liver 'fluke of sheep]. 



Accordingly the following conditions are necessary for the com- 

 pletion of the entire development : (i) The terminal host in which 

 the adult stage lives ; (2) an intermediary host into which the 

 miracidia penetrate and in which they become germinal tubes ; 

 (3) a second intermediary host in which the cercaria become en- 

 capsuled. In certain species this last stage is omitted, when their 

 cercaria encyst themselves, or, again (in other species), it may occur 

 within the first intermediary host, especially when the cercaria 

 (which in that case do not acquire the oar-like tail) do not 

 swarm out, but encyst themselves at once within their sporocysts. 

 The development, moreover, may be further complicated by rediae 

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

 first intermediary host and not in a fresh one. 



Animals that harbour adult digenetic Trematodes have probably 

 become infected by ingesting their encysted forms, which they have 

 .either swallowed with certain animals (secondary intermediary 

 hosts) on which they feed, or ingested with water or with plants, 

 or finally with the first intermediary host ; whereas animals fostering 

 encysted (digenetic) Trematodes have been infected by the corres- 

 ponding cercaria stages, and animals bearing germinal bodies (sporo- 

 .cysts and redia), Have been straightway attacked by the miracidia. 



Certain species of ducks and geese become infected with E chinos- 

 tomum echinatum by devouring certain water snails (Limnceus, Palu- 

 dina) in which the encysted stages occur. Oxen become infected 

 with Paramphistomum cervi (i.e., Amphistomum conicum) by swallow- 

 ing with the 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 cysts of the liver fluke are attached ; 



