TREMATODA. 161 



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 flukes 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 

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

 produce about half a million 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 must die. The free embryo 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 

 {Limnceus truncatulus or minutus), into which it bores its 

 way. In an epidemic among horses and cattle in the 

 Hawaiian Islands, the host was L. cahuensis. Within the 

 snail, e.g. i»M:he pulmonary chamber, the embryo becomes 

 passive, loses its cilia, increases in size, and becomes a 

 sporocyst. Sometimes this sporocyst divides transversely 

 (Fig. 72 (4)). 



Within the sporocyst certain cells behave like partheno- 

 genetic ova. Each segments into a ball of cells or morula, 

 which is invaginated into a gastrula, and grows into another 

 form of larva — the redia. These redise burst out of the 

 sporocyst, and migrate into the liver or some other organ, 

 killing the snail if they are very numerous. Indeed, the 

 death of the snail is probably necessary for the escape of 



