21 8 UNSEGMENTED WORMS 



posteriorly in a terminal vesicle opening to the 

 exterior. 



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. The ova pass from its tubes 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 afore- 

 said 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 flukes it is 

 said to be a copulatory duct ; in others it is regarded as a safety valve 

 for overflowing 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 

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

 produce about 50,000 embryos, which illustrates the 

 prolific reproduction often associated with the luxurious 

 conditions of parasitism, and almost essential to the con- 

 tinuance of species whose life-cycles are full of risks. 

 Outside of the host, but still within the egg-case, the 

 embryo develops for a few weeks, and eventually escapes at 

 one end of the shell. Those which are not deposited in 

 or beside pools of water soon die. The free embryo, 

 known as a miracidium, is conical in form, covered with 

 cilia, provided with two eye-spots, and actively locomotor. 

 By means of its cilia it swims actively in the water for some 

 hours, but its sole chance of life depends on its meeting 

 a small amphibious water-snail (Limnceiis truncatulus or 

 minutus), into which it bores. In an epidemic among 

 horses and cattle in the Hawaiian Islands, the host was 

 L. oahuensis ; in the same locality the host may be 



