358 ZOOLOGY. 



the egg-shells. From the shell-gland passes forwards a 

 much convoluted duct called the uterus, because the fully 

 formed eggs accumulate in it ; it passes forwards to open 

 on the ventral surface between the oral and the ventral 

 suckers. The testes are paired, but not symmetrical. 

 They are branched tubular structures like the ovary, occupy- 

 ing the middle region of the posterior two-thirds of the 

 body between the vitelline glands. Each testis gives off 

 a vas deferens which passes longitudinally forwards ; the 

 two ducts unite into a dilatation called the vesicula 

 seminalis. From this a fine short duct passes to the 

 penis, a muscular tubular organ surrounded by a sheath 

 called the cirrus-sac. In copulation the penis is everted 

 or evaginated and protruded from the mouth of the cirrus- 

 sac. The opening of the uterus lies close beside the opening 

 of the penis. A short straight canal called the Laurer- 

 Stieda canal leads from a dorsal pore to the junction of 

 the ovarian and vitelline ducts with the shell- gland. 



4. Production of Ova. The true ova are produced in 

 the ovary. Copulation has apparently not been observed, 

 but probably consists in the injection of spermatozoa from 

 another individual by the penis into the Laurer-Stieda 

 canal. This duct is known to contain spermatozoa, and 

 thus the ova are fertilised just as they reach the shell- 

 gland. Each ovum becomes surrounded by a large number 

 of yolk cells produced by the vitelline glands and brought 

 to the shell- gland by the vitelline duct. Then in the shell- 

 gland the ovum and its envelope of yolk are enclosed in a 

 tough chitinous egg-shell, and the complete egg then passes 

 into the uterus, finally to escape by the aperture of that 

 duct. The number of eggs produced is very large, as in 

 most parasites, where the probability of any single egg 

 escaping destruction and successfully developing into a 

 mature individual is small. 



5. Life-history the Ovum. The completely formed 

 eggs of the liver-fluke are extruded in large numbers from 

 the uterus through its aperture, then make their way 

 down the bile-ducts into the intestine of the sheep, where 



