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.— a. B, Mnnostomnm mntabile.—A. the ciliated emhryo (a) inclosinsr the 

 zooid, {b.) represented free in B (after Siebold) : C, Redia, or kine's yellow worm 

 of Distnma pacificum, containine grerms of other Redice ; D. Redia containine 

 CercaricE (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 As- 

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

 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. hepaticimi, and 3Io7iosto7ni(m mutabiley 

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

 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 3fonost07nu7n gives exit to a larva, having 

 the form of a cylindrical sac with two lateral prolongations 

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

 44, j&, (7), has a mouth and a simple c^ecal 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 



