Fig. 156.—Drawing from a microscopic preparation, showing a hæmorrhage in the parenchyma of the liver caused by the common liver fluke (Fasciola hepatica). a, Atrophic liver tissue; b, round-cell infiltration; c, a portion of the parasite; d, hæmorrhage. (After Schaper, 1890, Pl. I., Fig. 1.)
From the above we see that this parasite runs through three generations, namely:
(1.) Ovum, miracidium, and sporocyst ... first generation.
(2.) Redia ... second generation.
(3.) Cercaria and adult ... third generation.
During this curious development, which lasts about ten to twelve weeks, there is a constant potential increase in the number of individuals, for each sporocyst may give rise to several (five to eight) rediæ, each redia to a larger number (twelve to twenty) cercariæ, and each adult to an enormous number (37,000 to 45,000) of eggs. This unusual fertility of the parasite is necessary because of the complicated life history and the comparatively small chance any one egg has of completing the entire cycle.
Hosts. An interesting and, from an agricultural standpoint, an important matter connected with this fluke is that it is found in a large number (about twenty-five) of domesticated and wild animals, and this fact probably explains to some degree the wide geographical distribution of the parasite.
Fig. 157.—Drawing from a microscopic preparation, showing the glandular hyperplasia of the mucosa of a gall-duct caused by the common liver fluke (Fasciola hepatica). a, Hypertrophied submucosa; b, interstitial connective tissue; c, compressed lobule; d, lumen of the gall-duct—thickened fibrous wall of the gall-duct. (After Schaper, 1890, Pl. I., Fig. 2.)