THE LIVER-FLUKE. 153 



their life in some Invertebrate. The relations of the class 

 are on the one hand with the free-living Turbellarians, on 

 the other hand with the parasitic Cestodes or tapeworms. 



Type, The Liver-Fluke (Fasciola (or Distomum) hepaiica). 



The adult fluke lives in large numbers in the bile-duct of 

 the sheep. It also occurs in other domestic animals, and rarely 

 in man. In the sheep it causes the serious disease called 

 liver-rot. It is flat, oval, and leaf-like, measures about an 

 inch in length by half an inch across the broadest part, 

 varies from reddish brown to grayish yellow in colour. As 

 the word Distomum suggests, there are two suckers, — an 

 anterior, perforated by the mouth; a second, imperforate, 

 a little further back on the mid-ventral line. 



There is a muscular pharynx and a blind alimentary canal 

 which sends branches throughout the body. The nervous 

 system consists of a ganglionated collar round the pharynx, 

 from which nerves go forward and backward ; of these, the 

 two which run laterally are most important. Although the 

 larva has at first two eye-spots, there are no sense-organs 

 in the adult. The body-cavity is represented only by a few 

 small spaces. Into these there open the ciliated ends of 

 much branched excretory tubes, which unite posteriorly, 

 and communicate with the exterior by a terminal pore. The 

 reproductive system is hermaphrodite and complex. From 

 much branched testes, spermatozoa pass by a pair of ducts 

 (vasa deferentia) into a seminal vesicle lying in front of the 

 ventral sucker. Thence they are expelled by an ejaculatory 

 duct, which passes through a muscular protrusible penis. 

 The retracted penis and the seminal vesicle lie in a space or 

 " cirrus sac " between the ventral sucker and the external male 

 genital aperture. The ovary also is branched, but less so 

 than the testes. From its tubes ova are collected into an 

 ovarian duct. Nutritive cells are gathered from very diffuse 

 yolk glands, collected in a reservoir, and pass by a duct 

 into the end of the aforesaid ovarian duct. At the junction 

 of the yolk-duct and the ovarian duct there is a shell- 

 gland, which secretes the " horny " shells of the eggs, and 

 from near the junction, a fine canal (the Laurer-Stieda 



