Etiology. 



547 



coelium lanceolatiim). Both parasites are leaf-sliaped, elon- 

 gated and unprovided with appendages. The body is broader 

 towards the middle part and there are two suckers at the ante- 

 rior ends. 



Distomum hepaticum measures from IS to 31 mm. in length by 4 to 13 mm. 

 in width, and is covered with minute spines that point backwards. At the anterior 

 extremity there is a conical process which carries at its apex the oral sucker. 

 On the under surface, about 3 mm. behind this, there is a second and larger 

 sucker. The eggs (fig. 72) are brown or greenish-yellow, oval, and provided at 

 one pole with a small cap. This cap may be rendered visible either by exerting 

 pressure on the eggs or by the addition of potassium hydrate solution. The eggs 



measure 



130 to 145 fi in length and 70 to 00 ^ in width. 



Fig. 72. 



The Distomum lanceolatum Is only 4 to 9 mm. in length and 2.5 mm. broad. 

 It is slender and lancet-shaped. The eggs which are brownish in color are also pro- 

 vided with a polar cap (fig. 71) and measure 37 to 40 [x in length. 



Life-cycle of the Distomum Hepaticum. According to Weinland, 

 Leuekart, and Thomas, the life-cycle of the parasite is as follows : The 

 eggs of the mature flukes are passed out with the feces of the host 

 into the outer world. Provided segmentation has taken place, an em- 

 bryo develops under suitable conditions of temperature (at least ten 

 to twelve C.) and moisture in about three to six weeks. The em- 

 bryo escapes by raising the polar cap. It is elongated in shape and its 

 surface covering is composed of polyhedral cells and 

 thickly covered with cilia. These larvffi (mira- 

 cidium) are very actively motile in water. By means 

 of a movable spine at their anterior end they bore 

 their way into the body of certain fresh-water snai 

 The snails usually selected are Limnjeus minutus or 

 truncatulus, and more rarely L. pereger and others. 

 During the summer months they become converted 

 into sporocysts within two weeks. 

 Each germ-cell gives rise to five to 

 eight rediae, and each redia to fifteen 

 to twenty cercariae. The cercarige 

 pass out of the oral opening of the 

 rediae, and then leave the body of the 

 snail. The cercaria has a flat oval 

 body measuring 280 fx in length, 

 and 230 fx in breadth. It is also pro- 

 vided with a caudal appendage twice 

 that length. Two suckers and a bifid 

 intestine can already be distinguished 

 the water the cercaria work their way up grass-stalks and there secrete 

 a sticky, mucoid substance which serves both to encapsule them and to 

 cement them to the grass. In this condition the cysts measure about two 

 to three mm. If such cysts be ingested by certain animals the parasites 

 make their may up the bile-ducts and there become sexually mature, ac- 

 cording to Leuekart in three weeks, and according to Thomas in five to 

 six weeks. As a rule a proportion of the mature parasites die. The ma- 

 jority, after laying eggs, pass dowTi the bile-ducts and not rarely 

 accumulate in the gall-bladder. They then pass into the intestine, 

 where they promptly die and are digested. Only a few are passed into 

 the outer world in a healthy condition (Schaper). The eggs are passed 

 into the intestine with the bile, and are eventually expelled from the host. 



According to Gerlach, the period spent by the distomes in the liver 



Fig. 71. Egg of Distomum lanceolatum. 

 Fig. 72. Egg of Distomum hepaticum. 



After swimming for a time in 



