THE LIVER FLUKE 125 



LIFE-HISTORY 



The life-history of the Hver fluke is a remarkable and interest- 

 ing process (Fig. 88). The eggs, up to 3,500 a day, are laid in 

 the bile ducts of the sheep. So long as they remain within the 

 body of the latter they do not develop, but when they have 

 been carried by the bile to the intestine, and thence passed to 

 the exterior with the droppings, they will develop in damp 

 spots if the weather be warm, and the pH of the water be less 

 than 7.5, the optimum being about 6.5. In a few weeks a larva 

 known as the miracidium emerges. It is rather more than a tenth 

 of a milhmetre long, conical, covered with a layer formed by 

 five rings of big, ciliated cells, provided with two eye-spots, a 

 small gut, and two flame cells, and filled by a mass of cells. 

 It swims by means of the cilia, with the broad end forwards. At 

 this end there is a knob which can be thrust out as a conical spike. 

 If it can find a member of a particular species of water snail 

 known as LimncBU tnmcatula ^ it works its way into the tissues 

 of the snail, thrusting out its spike and rotating by means of its 

 ciha so as to bore in. Within the snail the ciliated cells are lost 

 and the larva increases in size and grows into a hollow sac or 

 sporocyst. Sometimes this multipHes by dividing transversely. 

 Within the sporocyst some of the cells lining the cavity behave 

 Kke fertihsed ova, dividing to form a blastula, which invaginates 

 to give rise to a two-layered sac or gastrula. These cells, however, 

 have not undergone fertilisation, and their development is an 

 example of parthenogenesis, the development of unfertilised ova. 

 The gastrula grows into another form of larva, the redia, which 

 bursts out of the sporocyst and migrates, usually into the liver 

 of the snail. The rediae devour the tissues of the snail and finally 

 kill it. Each redia has an elongated body with an anterior mouth, a 

 muscular pharynx, and a short, sac-hke gut. A little way behind 

 the pharynx the body-wall is thickened to form a muscular collar, 

 and not far from the hind end are two blunt conical processes 

 on one side. Posteriorly there is a large body cavity hned by an 

 epithelium like that of the cavity in the sporocyst. Cells derived 

 from the wall of the body cavity develop in much the same 

 way as in the sporocyst and give rise to daughter rediae, which 

 escape from the parent by an opening behind the collar. Several 



1 Other species and genera of water snail are sometimes infected in foreign coun- 

 tries, and occasionally L. stagnalis and others may be infected in Britain. 



