TYPE PLATYEELMINTHEa. 



149 



{gc), wliich are in reality germ-cells or ova capable of under- 

 going a parthenogen'etic development. Eventually this larva 

 makes its way into the interior of an animal of some kind, 

 usually a Mollusk, and there undergoes a further develop- 

 ment, either retaining its digestive apparatus and elongating 

 somewhat to form a Bedia (Fig. 79, B), or becoming an oval 



Fig. 79. — A, Ciliated larva, and B, Redia of Dzstomum hepaticum (after Leuok- 



ART). 



d = intestine. m = moutb. 



gc = germ-cells. r' = second generation of Redias. 



sac without mouth or digestive tract, the Sporocyst. The 

 Eedia is a much more highly organized form than the Sporo- 

 cyst and is frequently capable of motion, two blunt projec- 

 tions near the hinder end of the body serving as supports in 

 a somewhat similar' manner to the sucker-like feet of cater- 

 pillars. It adheres to the wall of a cavity of its host, from 

 which by energetic action of its muscular pharynx it is able 

 to absorb nutrition. 



From this stage onwards the development varies in com- 

 plexity in various forms. It is simplest in Monostomum muta- 

 bile, whose ciliated embryo, while still free-swimming, contains 

 within it a small sexually immature Monostomum, and after it 

 has made its way into the interior of its MoUuscan host the 

 young Monostomum becomes encapsuled in the tissues of its 

 host. The mode of origin of this immature form has not as 



