LIFE HISTORY OF LIVER FLUKE. 167 



which sends branches throughout the body. The nervous 

 system consists of a gangUonated 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 eye spots to start with, 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 is also 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 

 canal) seems to pass direct to the exterior, opening on the 

 dorsal surface. The meaning of this is still somewhat 

 uncertain. In some cases it is said to be a copulatory 

 duct ; in others it is regarded as a safety valve for over- 

 flowing products. From the junction of the ovarian duct 

 and the duct from the yolk reservoir, the eggs (now furnished 

 with yolk cells, accompanied by spermatozoa, and encased 

 in shells) pass into a wide convoluted median tube, the 

 oviduct or uterus, which opens to the exterior at the 

 base of the penis. Self fertilisation is probably normal, 

 but in some related forms cross fertilisation has been 

 observed. 



Life History. — The fertilised and segmented eggs pass in 

 large numbers from the bile duct of the sheep to the in- 

 testine, and thence to the exterior. A single fluke may 

 produce towards half a million embryos, which illustrates 

 the prolific reproduction often associated with the luxurious 



