294 



Diseases of the LtvfiR. 



the biliary ducts of oxen, sheep, and goats of two species of distomata, 

 viz., the Distoma hepaticum or Fasciola hepatica, and the Distoma lanceo- 

 latiim. 



In 1875 Zundel established the causative relation between the pre- 

 sence of distomata in the liver and the development of jDrogressive fatal 

 cachexia in most of the animals affected. This 

 B opinion was emphasised by the works of Leuckart 



and Thomas on the development of distomata, 

 and at the present day the parasitic theory is 

 accepted as beyond question. 



Fasciola hepatica {Distoma hepaticum). — The 

 common liver fluke of cattle, sheep, swine, etc. 



Life history. The adult jparasite, instead of 

 producing young similar to itself and capable 

 of developing directly into adults in cattle, pro- 

 duces eggs which develoj) into organisms totally 

 different from the adult form, living a parasitic 

 life in other animals. In scientific language, the 

 13arasite is subject to an alternation of genera- 

 tions, together with a change of hosts. The fol- 

 lowing summary of the life history will make this point clear: — 



(a) The adult hermaphroditic irorm (Figs. 144 and 145) fertilises itself 

 (although a cross fertilisation of two individuals is not impossible) in the 

 biliary passages of the liver, and produces a large number of eggs. 



(b) Eggs (Figs. 143, 146 and 147). — Each egg is composed of the 



Fig. 142. — Fasciola hejja- 

 tica. A, young ; B, 

 adult parasite. (After 

 Kailliet.) 



Fig. 143. — Eggs of Fasciola Jiepatica. A, 

 from the bile duct ; B, embryonic ; C, after 

 opening. (After Kailliet.) 



Fig. 144. — Common liver 

 fluke {Fasciola hepa- 

 tica), natural size. 

 (Stiles, Annual Report, 

 U.S.A. Bureau of Agri- 

 culture, 1901.) 



following parts : (1) A true germ cell, which originates in the ovary and is 

 destined to give rise to the future embryo ; (2) a number of vitelline or 

 yolk cells, which are formed in a specialised and independent portion 

 (vitellogene gland) of the female glands — instead of developing into embryos 

 the yolk cells form a follicle-like covering for the true germ cell, and play 

 an important role in the nutrition of the latter as it undergoes further 



