228 



PLATYHELMINTHES. 



in which case there is no intermediate host and no alternation of 

 generations : such forms are monogenetic, and are mostly ecto- 

 parasites; or they have the form of ciliated larvae (Mimcidium) 

 which pass into an intermediate host, and eventually become 

 transformed into the adult without alternation of generations : 

 such forms are monogenetic and metastatic, and are endoparasites ; 

 or they undergo a change of host and present the phenomenon of a 

 complicated alternation of generations, in which the miracidium larva 

 develops into an asexual form whicfc reproduces itself : such forms are 

 called digenetic, and are all endoparasites. In the monogenetic forms 

 the relatively large eggs become, in most cases, attached on the place 



FIG. 180. Embryonic development of Distomum tereticollc (after H. Schauinsland). a, egg 

 after hardening in picric acid ; Es shell ; E ovum ; D yolk-cells, b, the yolk has been largely 

 used up by the embryonic cells ; // the first-formed cells of the cellular membrane of the 

 shell ; S operculum. c, later stage, the shell membrane H surrounds the embryo, the yolk 

 D almost gone, d, later stage showing ectoblast EC, the internal mass being endoblast. 

 e, later stage. /, a ripe embryo just before the hatching ; B setigerous plates with their nuclei. 



where the mother lives, and the young are hatched out as non-ciliated 

 or partially ciliated forms (Polystomum integerrimum has transverse 

 rows of cilia at hatching, Fig. 186). In the case of the metastatic 

 and digenetic forms the eggs (relatively small in the Digened} are 

 deposited in a damp place, usually in the water. 



In the Digenea* the ovum is placed next the operculum the 

 pole of the egg where the anterior end of the future embryo will 

 be formed (Fig. 180, E). It undergoes a total cleavage and gives 

 rise to a solid cell-mass, from the surface of which is differentiated 

 a cellular membrane (Fig. 180, H), which lines the shell and is 



* H. Schauinsland, "Beitrag z. k. d. embryonalen Entwicklung der Trema- 

 toden," Jena, Zeitschrift, 16, 1883. 



