THE DEVELOPMENT OF THE TREMATODA. 



179 



a ciliated embryo from the ovum. This parasite is found in 

 the blood-vessels of man, chiefly in those of the urinary or- 

 gans, the ova escaping from the body through the ulcerated 

 surfaces to which the parent gives rise. In the ectoparasites, 



Fig. 44.-^4, 2?, Monostomum mutabile.—A, the ciliated embryo (a) inclosing the 

 zooid, (b,) represented free in B (after Siebold) ; C, Bedia, or king's yellow worm 

 of Distoma paciftcum, containing germs of other Redice ; D, JRedia containing 

 Cercarioe (a) ; E, Cercaria ; F, Distoma, which results from the metamorphosis 

 of the Cercaria. (After Steenstrup.) 



the embryo passes into a form identical with or closely resem- 

 bling that of the parent while still within the egg, as in -4s- 

 pidogaster. When this happens (e. g., Distoma variegatum, 

 D. tereticolle), the one end of the embryo is often provided 

 with spines, and it is capable of slow creeping movements. 

 But, in most of the endoparasites, the embryo leaves the 

 parent as a morula, which is usually ciliated. Thus, in Disto- 

 ma lanceolatum, D. hepaticum, and Monostomum mutabile, 

 the embryo which escapes from the egg has a ciliated invest- 

 ment, which propels it rapidly through the water, and may 

 be provided with eyespots and water-vessels (Fig. 44, A). 

 On becoming attached to the animal upon which it is parasit- 

 ic, the embryo of Monostomum gives exit to a larva, having 

 the form of a cylindrical sac with two lateral prolongations 

 and a tapering tail. The JRedia, as this form is called (Fig. 

 44, jB, C), has a mouth and a simple caecal intestine, but no 

 other organs. In its cavity a process of internal gemmation 

 takes place, giving rise to bodies resembling the parent in 

 shape, but destitute of reproductive organs, and furnished 



