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) 

 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 (Limnaus truncatulus or 

 minutus\ into which it bores. In an epidemic among 

 horses and cattle in the Hawaiian Islands, the host was 

 Z. oahuensis ; in the Sandwich Islands th host is 

 Z. peregra, in Victoria Bulimus tenuistriatus. This 

 diversity of host, also remarkable in the adult, is very 

 unusual. Within the snail, e.g. in the pulmonary chamber, 

 the embryo becomes passive, loses its cilia, increases in 

 size, and becomes a sporocyst. 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 redia. These redise burst out of the 



