THE TREMATODA 



73 



The miracidium (Fig. XVI.), once set free, swims about seek- 

 ing for a definite mollusc, into which it will bore its way by 

 means of the snout. Wagener (1857) was the first to observe the 



n 



FIG. XVI. The life-history of Distomum hepaticum. (After Thomas.) 



1. The free swimming larva, or "miracidium," showing external appearance. 



2. Sporocyst, containing germ balls and young rediae. 



3. A young redia, containing germ balls ; the enteron is shaded but unlettered. 



4. A fully formed redia, containing a daughter redia, two cercariae, and germ balls. 



5. A free cercaria. 



a, head papilla ; ft, anterior ring of ectoderm cells ; c, eyes ; d, same, degenerating, in 

 sporocyst ; e, embryo, at gastrula phase, in sporocyst ; /, enteron of young redia ; g, pharynx ; 

 h, collar of redia ; i, lip ; j, oesophagus of cercaria ; k, " germs," at blastosphere stage ; I, paren- 

 chyma ; m, posterior, locomotive processes of redia ; n, germ cells, in wall of redia ; o, birth 

 opening ; p, young cercariae in redia ; q, daughter redia in redia ; r, circumoral sucker of 

 cercaria ; s, ventral sucker ; t, cystogenous cells. 



entrance into a mollusc in the case of D. cygnoides. In the case 

 of D. hepaticum this " intermediate host " is Limnaeus truncatulus, 

 as was proved by Thomas (42). The animal makes its way into 

 the liver, and undergoes a degeneration a result no doubt of its 



