THE DEVELOPMENT OF THE TREMATODA. 



181 



in which it remains quiescent, but undergoes some further 

 advances in development, the coronal hooklets making their 

 appearance. 6. When a Paludina^ thus infested, is swal- 

 lowed by a water-bird and digested, the cysts are set free in 

 the alimentary canal of the bird ; sexual organs appear within 

 the included Distoma ; the body elongates and narrows an- 

 teriorly ; the sucker moves nearer the head, and the coronal 

 circlets reach their full development. The Distoma gradually 

 assumes the form of the parent, attaches itself by its hooklets 

 to the intestinal walls, and acquires complete sexual organs.^ 

 Thus the developmental stages of Distoma inilitaj^e in-a,y be 

 summed up, as : 1. Ciliated larva. 2. Hedia. 3. Cercaria. 



4. Cercaria, tailless and encysted, or incomplete Distoma. 



5. Perfect Distoma. 



The stages of transition vary in different genera. Thus, 

 several generations of Bedioe may intervene between the 



Fig. 45.— Bncephalus po^ymorphus of the fresh-water muscle.—^, ramified sporocyst ; 

 B, portion of the same mure ma,:;nided: a, outer coar, 6, iuuer ; c, d, serrn- 

 masses in course of development ; C, tme of the germ-masses more liighly mag- 

 nified ; Z>, Bucephalus ; a, 6, suckers ; c, clear cavity ; rf, caudal appendages. 



third and fourth stages ; or the mature animal may appear at 

 the close of this stage, having undergone no Cercarian meta- 

 morphosis. 



In Bucephalus polymorphiis, a parasite of the fresh- 

 water muscle (1^'ig. 45), two caudal appendages, which seem 

 to correspond with the tail of the ordinary Cercarim, become 



1 Van Beneden, " Memoire sur les Vers Intestinaux.' 



