CHAPTER XVII. 



THE LIVER-FLUKE AND TAPEWORM. 



THE Liver-fluke (Fasciola (Distoma) hepatica), a parasitic 

 organism which is the cause of the liver-rot in sheep a disease 

 which, during the winter of 1879-80, caused in England the death 

 of some three million sheep is a flattened oval animal about 

 an inch in length, and provided on its ventral surface with two 

 suckers, from which it received its original name of Distoma. 



General Summary of Life-history. The liver-fluke, as such, 

 is found in the liver of sheep. Here it reaches sexual maturity, 

 each individual producing many thousands of eggs, which pass 

 with the bile into the alimentary canal of the host, and are dis- 

 tributed over the fields with the excreta. Here, in damp places, 

 pools, and ditches, free and active embryos are hatched out of the 

 eggs. Each embryo (Fig. 97, C., much enlarged) is covered with 

 cilia, except at the anterior end, which is provided with a head- 

 papilla (h. p.). When the embryo comes in contact with any 

 object it, as a rule, pauses for a moment, and then darts off 

 again. But if that object be the minute water-snail, Limnceus 

 truncatulus (Fig. 97, B., nat. size), instead of darting off the em- 

 bryo bores its way into the tissues until it reaches the pulmonary 

 chamber, or more rarely the body-cavity. Here its activity 

 ceases. It passes into a quiescent state, and is now known as 

 a sporocyst (Fig. 97, E.). The active embryo has degenerated 

 into a mere brood-sac, in which the next generation is to be pro- 

 duced. For within the sporocyst special cells undergo division, 

 and become converted into embryos of a new type, which are 

 known as redice (F.), and which, so soon as they are sufficiently 

 developed, break through the wall of the sporocyst. They then 



