TREMATODES : DEVELOPMENT 225 



hatch in water, where they move with the aid of their cilia. Sooner 

 or later they penetrate into an intermediate host, which is always 

 a snail or a mussel, and while certain of their organs disappear, 

 they grow into a gutless germinal tube (SPOROCYST, fig. 131). These 

 are simple elongated sacs with a central body cavity. They may or 

 may not have excretory tubules. In these, according to the species, 

 the larval stages (CERCARIAE) that will ultimately become adult worms 

 are produced, or another intermediate generation is first formed, viz., 

 that of the REDi^E 1 (figs. 132, 133), which are always provided with an 

 intestine, and these then give rise to cercariae (figs. 130, 134). The 

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

 with the assistance of their rudder-like tails. After a little time, how- 

 ever, they usually again invade an aquatic animal (worms, molluscs, 

 arthropods, fishes, amphibians), then they lose their tails and become 



FIG. 130. A group of cercarioe of Echinostoma sp. (from 

 fresh water). 25/1. 



encysted (fig. 135) ; here they wait until they attain, together with 

 their host, the suitable terminal host, and in this new situation they 

 establish themselves and reach maturity. Or, again, the cercariae may 

 themselves encyst in water or on foreign bodies (plants) and wait until 

 they are taken up directly by the terminal host, e.g., 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 intermediate host into which the 

 miracidia penetrate and in which they become sporocysts ; (3) a 

 second intermediate host in which the cercariae become encysted. 

 In certain species, as in Fasciola Jiepatica, this second host is omitted, 

 as the cercariae spontaneously encyst on plants, or again (in other 

 species) encystment may occur within the first intermediate host, when, 



1 [In Fasciola hepatica in the summer months the rediae give rise to daughter redioe, which 

 then give rise to cercariae. J. W. W. S.] 



15 



