410 The Animal Kingdom 



The adult of Fasciola hepatica is a flattened worm found in the bile 

 duct of sheep. This arrowhead-shaped animal, 20 to 30 mm. in length, 

 is constantly producing eggs which pass down the bile duct into the 

 intestine, and out with the feces of the host. These eggs have heavy 

 covers which resist the action of the digestive enzymes of the host and 

 also are able to resist considerable drying action once they have reached 

 the ground. 



METACERCARIA 



CERCARIA V^;;y "tulA VJ^' S''°''°CYST '^' MIRACIOIUM 



Fig. 135.4. — The life history of the sheep liver fluke, Fasciola hepatica. 



Development proceeds in the egg only at temperatures above SC F. 

 At lower temperatures, however, the egg will remain viable for several 

 months. In warm, moist soils, the egg develops into a larval form in 

 approximately ten days. This larval form, the miracidium, is ciliated 

 and has two eyespots and a pointed anterior end. Nerve ganglia and 

 protonephridia are present. This larva is free swimming, moving about 

 in the water until it finds a suitable snail of the genus Lymnaea. If a 

 suitable host is not encountered within eight hours, the miracidium dies. 

 Once this host is found, however, the miracidium burrows into the soft 

 tissues and enters the pulmonary chamber or lymph vessels. Here it 

 loses its cilia and becomes a saclike structure, the sporocyst. Within the 

 sporocyst, germ cells develop parthenogenetically into the next stage. 



