TREMATODA. 



231 



which the different stages (miracidium, sporocyst, redia, encysted form, 

 sexually mature animal) of the digenetic Trematoda are buried. 



The transitions from one host to another are effected partly by 

 independent migration (Miracidium, Cercaria), partly ]>y passive 

 migration (encysted sexless adult). Modifications of the ordinary 

 course of development may, however, take place; these may be 

 either complications or simplifications. The embryo at hatching 

 may contain a single redia (as in Monostomum flavum and mutabile), 

 which it carries about till it enters the first host (Fig. 184, I). In 

 some cases the course of development is simplified by the omission 

 of the second intermediate host, viz., that which contains the 



encysted immature 



Trematode (Cercaria 



macrocerca of Dis- 



tomum cygnoides; 



also Leucochloridium, 



the sporocyst of Dis- 



tomum macrostomum, 



in the tentacles of 



Succinea ampJiilia ; 



D. hepaticum, in 



which the Cercaria 



encysts upon a blade 



Of grass). In other FlG . 184 ._a, Miracidium (ciUated 



cases the sporocysts 



FIG. 183.-A sporocyst produce rediae, which 



with contained Cerca- 



riae c, B boring spine produce Cercariae or 



of Cercariae. 



embryo) of Diplodiscus (Amphi- 

 stomum) subclavatus (after G. 

 Wagener). D intestine ; Ex ex- 

 cretory vessels. 5, Miracidiun- 

 of Monostomum, mutabile (after 

 v. Siebold). p eye-spot ; R redia. 



more rediae (D. he- 

 paticum). 



Further, there are unencysted young Distoma which never be- 

 come sexually mature in their host (e.g. in the lens and vitreous 

 humour of the vertebrate eye, in the jelly of Coelenterates). On 

 the other hand encysted forms are known (Distomum agamos of 

 Gammarids) which are sexually ripe and produce eggs within the 

 cyst (? self -impregnation). The sporocyst may increase by division 

 (e.g. the sporocyst of Cercaria minuta). The tail of the Cercaria 

 may become transformed into a sporocyst, and after detachment 

 produce a brood. 



Many forms possess great adaptability to changed conditions of 

 life, e.g. D. echinatum, which proceeds from Cercaria echinata of 

 Paludina vivipara and normally infests the intestine of the Duck 



