THE DEVELOPMENT OF THE TKEMATODA. 



It!) 



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 givos rise. In the ectoparasites, 



PIG. 44. A, .B, Sfonostomum mvtaMle.A, the ciliated emhryo (a) inclosing the 

 zof>irt (ft.) represented free in B (after Siebold) : C, Rtdia^ or king's yellow worm 

 of DixtnuHi /><!</ fl'citm, containing germs of other Redte ; D, Redia containing 

 Cercaria! (a) ; E, Cercaria ; F, Diftoma, which results from the metamorphosis 

 of the Cercaria. (After Steenetrup.) 



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

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

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

 D. tereticolle), the one end of the embryo is often provi<l< <1 

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

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

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

 ma lanceolatum, D. hepaticiim, and Monostomitm muta /'/>. 

 the embryo which escapes from the egg has a ciliated inv->t- 

 ment, which propels it rapidly through the \\;>t. r, and may 

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

 On becoming attached to the animal upon wlm-h it is parasit- 

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

 the form of a cylindrical sac with two lateral prolongations 

 and a tapering tail. The Jledio, as tins form is called (Fig. 

 44, j#, (7), has a mouth and a simple csecal intestine, but no 

 other organs. In its cavity a process of internal gemmation 

 takes place, giving rise to bodies resembling tho parent in 

 shape, but destitute of reproductive organs, and furnished 



