The science of life : fully illustrated in tone and line and including many diagrams

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

—any one individual having both male and female organs—but nevertheless they fertilize each other. Like most parasites, they are incredibly prolific. Their microscopic eggs are carried by the bile down the bile-duct of the sheep to the intestine ; thence, among the feces, they reach the exterior. During the next spell of warm, moist weather the hard egg-shell breaks and there emerges a creature very unlike its parents. This creature, the Miracidium, has a tiny conical body covered with cilia, two eyes of very elementary structure, and no mouth or stomach. Begotten by parents over an inch long, it is itself less than one twohundredth of that size. Nevertheless, it is active from

the moment of

hatching. It ie swims about Pe

in the puddles or ditches ‘or |

larger pools in wi the grazing meadows, or even through the thin surface film of water left on the grass after

Waitin Stage

hale A Wak ee ° rs, 3

Adult Flukes in Sheep

NAS Mh

showing a particular preference for the liver. These ingenious parasites, the redie, live and die and reproduce themselves inside the snail, weakening it but prudently not killing it, and in this way there follow several generations more, all very much alike. But at the chill of the following autumn there is another change. New forms appear in the little community—forms called cercarie, somewhat smaller than the redie, with big round heads and long thin tails. The cercarie are not satisfied by the leavings of their parents andplatyhelminth pilgrim fathers, so to speakthey bore their way out of the snail’s body in quest of a larger organism to colonize. They swim up wet blades of grass or the leaves of

orgestive Tube

Eqgs of

J. Flukes

RAR Ua

Livet x Sa, bo —

rain or dew,

with every ap- SSS = pearance of eS eager purpose. eS Now, there - Se are certain = Cercariae—=7 74 L kinds of snail eumaring 0

which live in = a= ponds or : among the damp grass in Swampy places, and it is for one of these that the little creature is searching. Its eagerness is excusable, for unless it succeeds in its quest within eight hours, it perishes. But if it does succeed, it bores its way into the body of the snail and alters in shape, becoming a long, hollow bag. At this stage it rests, nourished by the snail’s blood and growing slowly, for about a fortnight; then it reproduces itself{—it gives rise to a number of embryos and dies. That is the first generation. The offspring of this little adventurer differ both from their immediate parent and from the original flukes. They are elongated and wormlike, about a twenty-fifth of an inch long when fully grown, and they wander about the body of the snail and consume it,

Fig. 93.

Rediae in Po The Life-cycle of the Liver-fluke.

19 WE eSr-.

nd Snail

other herbage, and there they fortify themselves; they exude a slime which thickens into a hard white capsule and, thus protected, they wait. Ifnothing more happens to them they eventually die, but if the grass on which they are waiting chances to be swallowed by a browsing sheep (or other grazing animal, for flukes of the same species have been found in oxen and deer and even rabbits) they make good use of their opportunity. ‘Their capsules are obligingly digested away by his gastric juice, and when he has freed them in this way they creep up his bile-duct to his liver, where they thrive and grow at his expense. In about six weeks’ time they are ready to beget eggs to start on the adventurous cycle again. ‘There is, then, a perfectly

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