186 UNSEGMENTED “WORMS.® 
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) y 
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 (Zimmneus 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 Sandwich Islands the host is 
L. peregra, in Victoria Bulimus tenuistriatus. This 
diversity of host, also remarkable in the aduli, is very 
unusual. Within the snail, ¢.g. in the pulmonary chamber, 
the embryo becomes passive, loses its cilia, increases in 
size, and becomes a sforocyst. The sporocyst is a hollow 
sac, with a slightly muscular wall and with the beginnings 
of an excretory system. Sometimes this sporocyst divides 
transversely (Fig. 97 (4)). 
Within the sporocyst a few 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 vedia. These rediz burst out of the 
