154 WORMS. 



canal) seems to pass direct to the exterior, opening on 

 the dorsal surface. The meaning of this is still somewhat 

 uncertain. In some cases it is said to be a copulatory duct ; 

 in others it is regarded as a safety valve for overflowing pro- 

 ducts. 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.— T\\& 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 towards half a million embryos, which illustrates 

 the prolific reproduction often associated with the luxurious 

 conditions of parasitism, and almost essential to the continu- 

 ance of species whose life-cycles are full of risks. Outside of 

 the host, but still within the egg-case, the embryo develops 

 for two or three 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 is 

 actively locomotor. By means of its cilia it swims actively 

 in the water for some hours, but its sole chance of life seems 

 to depend on meeting a small amphibious water-snail 

 {Limnceus truncatulus), into which it bores its way. Within 

 the snail, e.g., in the pulmonary chamber, the embryo be- 

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

 a long sac or sporocyst. Sometimes the sporocyst divides 

 transversely, but as this is rare, we may leave it out of account. 



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 rediae burst out of the 

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

 ing the snail if they are very numerous. Each redia is a 

 cylindrical organism with a short alimentary canal. 



Like the sporocysts, the rediae give rise internally to more 

 embryos, of which some are simply redife over again, while 

 the last set are quite different, — long-tailed cercarice, with 



