Their main problem in life is ensuring that their young manage to attain an equally ideal position within another body of the same kind. That will not happen if the eggs remain alongside the adults, for few host species are cannibals, eating the bodies of their own kind. The eggs must somehow leave the host, and the easiest and most obvious way to do that is with the host's droppings. But even this cannot be a solution for all, for only a minority of mammals and birds are so insanitary that they become sioled by the dung of their fellows.
The solution to the problem provided by evolution is to recruit a different kind of host. A tapeworm living in the gut of a cat uses a mouse. Its eggs are shed with the cat's droppings. These may fall among grain or some other potential mouse food where they may be inadvertantly consumed by a mouse. The eggs, inside the mouse's body, hatch into larvae which migrate into its liver. There they form cysts and multiply into large numbers of yet a different form of lava. If the mouse is then caught and eaten by another cat, then another generation of tapeworm has succeeded in finding a new home.
Such a circuit is, of course, a very chancey one. Since the mouse is not seeking cat droppings, the vast majority of them will remain untouched. Even if the mouse does happen to eat a smear of one, the parasite's future is still far from assured. If the mouse is caught, not by another cat but by a dog or a fox or an owl, then the larvae within it will live no longer. Maybe only one in a hundred thousand will have the good luck to reach another cat. For this reason, internal parasites produce their eggs in astronomical numbers. A mature tapeworm, living in a human gut and needing its larvae into the flesh of a pig in order that another pork-eating human being will be infected, may shed a million eggs a day and over its life-time produce as many as seven thousand million of them.
(If you don't have much time start reading here for "mindcontrol" info)-DL
A few of these internal parasites have developed ways to improve their chances of completing the hazardous connections in their life life-cycle. In northern Europe many small birds, such as flycatchers and thrushes, carry flukes within their gut.
The parasites' eggs fall to the ground in the bird's droppings where they may be eaten by a grazing snail. Inside the snails body, they hatch into small actively swimming larvae that bore their way through the gut wall and into the liver. There they reproduce themselves and form little mobile cysts which make regular journeys every morning into the snails tentacles. These are normally thin, but when the parasite forces its way into them they become thick and club like. Not only that, but the stretched wall of the tentacle becomes so thin that it is transparent and the parasite within is easily seen. It is brilliantly coloured, banded with yellow, orange and dark brown.
To make itself even more conspicuous, it pulsates. The presence of the parasite for some reason also changes the snail's behaviour. Instead of returning to the safety of the leaf litter soon after dawn, as uninfected snails do, it remains out in the open for much longer. The throbbing coloured bands within the swollen tentacles quickly attract foraging birds. Perhaps it is their resemblance to caterpillars on which many birds regularly feed. Whatever the reason, the birds flutter down, peck the tentacles off the unfortunate snail and swallow them.
Once again, a new generation of parasites has managed to reach the same kind of safe home as that in which its parents flourished.
