706 Journal of Agriculture, Victoria. [10 Dec, 1917. 



bile duct into the intestine of the host, and thence to the exterior 

 (generally about Febraary). Here, if the necessary conditions of mois- 

 ture and moderate warmth (about 77° Fahr.) are present, the embryo 

 begins to develop within the egg shell. From the latter there escapes in 

 about two or three weeks' time a tiny conical ciliated larva with a small 

 papilla (or boring apparatus), at the broad anterior end. The lower 

 the temiaerature the more slowly do the eggs hatch out, but apart from 

 this, eggs laid at the same time vary considerably in the period required 

 for hatching. This fact is of much practical importance, since damp 

 ground over Avhich eggs have been scattered may he dangerous for a 

 long period independent of temperatiu-e conditions. 



This ciliated larva swims about in the water or moisture until it 

 meets with a particular species of fresh-water snail,, but it dies if it fails 

 to find this new intermediary host within a few hours (generally eight). 

 The embryo usually bores its way into the breathing chamber of its snail 

 host, and then undergoes development, first into a shapeless sac — the 

 sporocyst — this stage being reached by the end of a fortnight in warm 

 weather. 



Within this sac another generation is i^roduced known as the Redia, 

 Vi-hich is more complicated than the very degenerate sporocyst from each 

 of which some five to eight Kedia; burst their way out. The Eedise leave 

 the breathing chamber in which they are formed, and force their way 

 into various organs of the snail, especially into the liver, causing thereby 

 considerable injury to the snail tissues, so that a moderately heavy infec- 

 tion will soon kill the snail. 



The Rediaj may reach a length of 1-lSth of an inch, and has a simple 

 alimentary canal by which it digests the liver cells of its snail host. 

 Within the Redis are formed (1) daughter Redias, especially in summer- 

 time, and also (2) a third generation known as the Cercaria, to the 

 number of from twelve to twenty. A fall of temperature seems to hasten 

 the formation of the Cercaria. The fully-formed Cercaria escapes by 

 a birth-opening from the Redia, and can move about very rapidly in 

 moisture by its suckers or tail, as the case requires. 



Usually the body of the Cercaria is about l-80th of an inch in length, 

 oval or heart-shaped, and with. a long contractile tail. It has two 

 suckers, one oral, the other ventral, a simple forked intestine, and certain 

 cyst-forming cells. The.se Cercarise usually leave the body of the snail 

 and swim about for a while in the water, but soon they begin to enclose 

 themselves in a tiny snow-white cyst on leaves of grass or other substances 

 in the water, the tail being cast off. (In Australia some types of 

 Cercarise become encysted within the snail-host, evidently an extra pro- 

 vision against drought conditions. Whether the Cercarise of the liver- 

 fluke does so also is not known, though it is probable.) Should this cyst 

 be eaten by a sheep or other herbivorous animal, either on grass or witbin 

 the snail, the cyst wall is dissolved in the stomach of this new final host, 

 and the Cercaria or young fluke is set free. 



It was foi-merly thought to wander up the bile-duct, and so into the 

 bile passages in the liver, but recent investigations prove that, in many 

 cases at least, invasion of the liver takes place by means of the circula- 

 tory .system after the setting free of the young fluke in the stomach, e.g., 

 flukes have been found in the liver of unweaned lambs and calves, and 



