70 MORPHOLOGY OF INVERTEBRATE TYPES 



number of cercarm or larvae of the third generation. The 

 cercaria is already similar to the fluke except that it has a tail 

 and that its reproductive organs are not yet developed. The 

 cercariae leave the snails, swim for a while in water with the 

 aid of their tail, sooner or later climb up a grass-blade, lose the 

 tail and become encysted. If now swallowed by a sheep, the 

 young fluke becomes liberated from its cyst, travels up the 

 gall duct and soon matures. In the case of Fasciola hepatica 

 a single egg may produce on the average ten rediae and each 

 redia forty cercariae. Thus a single egg will produce about 

 four hundred adult flukes and the progeny of a single fluke reach 

 the considerable number of 400,000,000 (four hundred million). 

 This immense productivity counterbalances the loss incurred by 

 the species in the passive transmission to the intermediary and 

 final host. 



Instructions 



1. Examine a prepared slide of D. lanceatum under the dis- 

 secting microscope to get a general view of the various organs. 

 Place now the slide under the microscope (50 diameters), 

 and make a full page drawing showing all organs. The 

 drawing ought to show the digestive system including mouth 

 sucker, pharynx, oesophagus and intestine; acetabulum or ven- 

 tral sucker; the common excretory canal with the excretory 

 opening at posterior end of animal; the male reproductive organs 

 including both testes, vasa deferentia, cirrus pouch with ejac- 

 ulatory duct in which one may sometimes distinguish the ves- 

 icula, the prostata portion, the prostata and the cirrus; the fe- 

 male reproductive organs including ovary, receptaculum seminis, 

 oviduct, yolk glands, longitudinal and transverse yolk ducts. 

 (In some cases Laurer's canal, the common yolk duct, the duct 

 of the receptacle and Mehlis' gland will also be visible.) Label 

 all parts including male and female genital openings. 



2. Examine under low power (50 diameters) the series of 

 cross-sections through D. lanceatum. Pick out for special study 



