]o2 A. P. THOMAS. 



be infected with the larval forms of the liver-fluke; and owing 

 to the habit this particular snail has of living so much out of 

 water, either on the banks of ditches or further away towards 

 the centre of the fields, if they are damp enough, the cercarise 

 will, on leaving their host, encyst on the grass in the places where 

 they have the best chance of being transferred to the herbivorous 

 mammals grazing on the ground. Having thus gained a suit- 

 able home they will attain the mature sexual condition, and 

 reproduce their species by means of ova, thus completing the 

 developmental cycle. 



Man himself sometimes serves as host to the liver-fluke, and 

 in this case the cysts are probably eaten with water-cress. 



4. Growth of Sexual Fluke. — From observations,^ which 

 I need not describe here, it appears probable that six weeks 

 elapse from the time of the entrance into the ultimate host 

 before the fluke begins to produce eggs. During growth the 

 body undergoes a very great change in form ; the posterior part, 

 which contains the reproductive organs, far outstrips the ante- 

 rior part (figs. 24 — 26). The ventral sucker shares in some 

 degree the greater growth of the hinder portion of the body ; 

 in the cercaria the suckers are of nearly equal size, and thie 

 same was the case in a young fluke 1"1 mm. in length. But 

 in specimens 2 — 3 mm. long, the diameters of the oral and 

 ventral suckers have usually the ratio of 1 : 1-1, and in still 

 larger examples 6 — 8 mm. long, the ratio is 1 : 12, whilst in 

 the adult the ratio is 1 : 1"35, though there is much individual 

 variety. 



The smallest fluke I have yet found in the liver of a sheep is 

 represented in fig. 23; the digestive tract, which in the cercaria 

 was simply forked, has already acquired a large number of 

 branches, though they are comparatively simple as yet. They 

 subsequently attain a much more complex form, owing to the 

 number of secondary branches. This branched intestine is 

 highly characteristic, and affords the principal reason for sepa- 

 rating the three species which constitute the genus Fasciola 

 from the species forming the distinct genus Distoma, none of 

 > 'Jouni. K. A. S/ 1881, p. 25. 



