The Fluke
Par Jean Painlevé
Nature’s foresight is bounded only by the unnecessary complexity which it creates for itself. As evidence, consider the incredible story of the sheep-liver fluke
The adult form, which lives in the liver of these delightful woolly ruminants, is like a flat bottle with a suction cup. They block the sheep’s biliary canaliculus, and cause fatal liver rot. Here, intricacy rubs shoulders with complexity. The dead sheep’s guts are eaten by a dog, or a wolf in the Romantic era (if there is no starving dog we’ll have to find one, or I won’t be able to finish my little story). Once the dog has had its fill, the rest will follow: as the French rhyme goes, the dog will bite the little goat, Biquette, who in turn wants to munch his cabbages.
Along with the sheep guts, the dog also swallows fluke eggs, which come out with its excrement in the form of elongated ciliate embryos with a rostrum or beak-like projection, an excellent tool for perforating a freshwater mollusc, the lymnea snail. (All of these bucolic scenes are played out in a swampy pasture where fresh stream water flows). The embryos reach the lungs where, while shedding their cilia, they learn to count - 31-32-33 - penetrating the interior of the snail before it rains. Each embryo grows in the form of a sac, inside which masses appear which, when they have budded into rediae, will fill it completely. Then, following the same process, these rediae produce more rediae, which in the hot season eventually give rise to around twenty cercariae: they are no longer crude sacs but are beginning to resemble the adult form, having developed a tail. This tail enables them to swim after escaping through the egg orifice of the covering membrane which is all that is left of the rediae. On reaching the riverbank, the cercariae settle on the grass, shed their tail, which is useless for climbing trees, and change into cysts - a very handy way of telling the world to get lost when it annoys you.
All that remains to be done is to wait patiently until they are swallowed by a sheep. Then when it meets the sheep’s gastric juices, the cyst membrane breaks down, releasing an adult fluke. And with that the process has come full circle.
Man’s shrewdness, a bit of a sneaky trick, lies in sending sheep which are only susceptible to a particular distoma to an area where the fresh water does not contain the molluscs which act as its go-between. The distoma, however, has endless tricks up its sleeve to complete its lifecycle regardless; some are born aviators, changing into cysts on a snail tentacle which they cause to swell and take on the colour of a tasty berry, so that a bird will be duped into swooping down to snaffle this fine morsel…
Jean Painlevé
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