94 



the shell gland which surrounds the junction of the ovarian 

 and vitellarian ducts. From an opening on the dorsal 

 surface a short Laurer's Canal (with a sperm reservoir) 

 passes to the aforementioned junction : its significance i 

 not clearly understood. 



Describe the Life- History of the Liver-fluke, Fasciola (syn* 

 Distomum) hepatica. 



The eggs passed out in the faeces of the sheep, are spread 

 about the pastures; and after a few weeks, in wet situa- 

 tions, the miracidia hatch out. The miracidium is a minute 

 conical ciliated embryo with a simple gut, a cerebral 

 ganglion with two simple eyes, and two flame- cells with 

 excretory vessels. It swims about for a time ; then it 

 enters a freshwater snail ( Limnaeus truncatulus) which is 

 common in pools and marshy fields ; and, in the snail's 

 mantle cavity, it becomes a sac - like sporocyst (without 

 internal organs). 



Germ cells, which were formed from the internal lining 

 of the body wall of the miracidium, develop in the sporo- 

 cyst (paedogenesis) into germ balls or morulae ; and 

 these develop into rediae, forms with a simple gut and a 

 " birth-pore." About six rediae emerge from each sporo- 

 cyst ; they bore into the liver or other parts of the snail, 

 and they produce (paedogenesis) the cercariae. In the- 

 summer months, the rediae develop a second generation 

 of rediae from which the cercariae are produced. About 

 fifteen cercariae emerge (by the " birth-pore ") from a 

 redia. 



The cercaria has the internal organisation of the adult 

 fluke ; it has also a tail for active locomotion. It leaver 

 the intermediate host ( Limnaeus) and attaches itself to 

 some plant or blade of grass, then casts off the tail and 

 encysts. This stage is attained about the end of s-ummer. 

 When the grass is eaten by the sheep, the young flukes 

 escape from their cysts and force their way into the bile- 

 ducts where they become adult. 



This life-history illustrates metagenesis or alternation of 

 generations ; but it is an alternation of a true sexual 

 generation, not with asexual but with sexual generations 



