TREMATODES : DEVELOPMENT 14! 



of an intermediary host which has ingested with its food the ova 

 escaped from the [primary] host. 



(5) The post-embryonic development of the Trematodes is accom- 

 plished in various ways ; the process is the most simple in the 

 ecto-parasitic species, the young of which are certainly to be 

 regarded as larvae, because they possess characteristics that are 

 lacking in the adult worms (cilia, simple gut, &c.), but which, 

 nevertheless, pass into the adult state direct, after a relatively 

 simple metamorphosis (Monogenea). In the Holostomidea that are 

 found chiefly in the intestine of aquatic birds, and which rarely 

 occur in other vertebrates, the ova develop in the water. The young 

 are ciliated all over, and, after having entered an intermediary 

 host living in the water (hirudinea, molluscs, arthropoda, amphibia, 

 fishes), they undergo a metamorphosis into a .second larval stage ; 



FIG. 76. A group of Cercaria of Echinostomum sp. (from fresh water). 25/r. 



they then encyst and await transmission into the final host, where 

 they become adult (metastatic Trematodes). 



In the remaining so-called digenetic trematodes one or two 

 intermediate generations interpose between the larval and ter- 

 minal stage, so that quite a number of adult animals may originate 

 from one egg. Usually the young, which are termed MIRACIDIA I 

 (fig. 75), hatch in the water, where they move with the aid of 

 their cilia. Sooner or later they penetrate into an intermediary 

 host, which is always a snail or a mussel, and while certain of 

 their organs disappear, they grow into an intestineless germinal 

 body (sporocyst, fig. 77). In these, according to the species, the 



[Also known as ciliated embryos. F. V. T.] 



