One of the most extraordinary and beautiful phenomena of nature, extraordinary because it is so ordinary and so close to us, is the sexual life of the common or, in the literal sense of the term, garden slug. Slugs are hermaphrodites and so possess both male and female organs. When they mate each slug penetrates and fertilizes the other. But they do it in the most astonishing way. One of them extrudes a rope of shiny mucus perhaps a yard long and on this they hang down from a twig or branch to perform their mating rituals in mid air. Foreplay can go on for hours as each partner slowly dances round the other, caressing each other, nibbling each other and savouring each other’s mucus. Eventually they penetrate each other and then either bite off each other’s penis or bite off their own – which organs are sometimes half their own length and corkscrew in design, so it is impossible to get them out of the vaginas in which their sluggish raptures have so recently occurred – so that they are now fully female. They then lay their beautiful translucent eggs under a leaf or stone. Recently the Heart of Borneo Project has discovered one hundred and twenty three new species, including a slug that fires a ‘love dart’ loaded with hormones with which it injects a mate.
The trematode parasite Dicrocoelium dendriticum spends the adult stage of its life cycle in the livers of cows and sheep. There they lay eggs which exit their hosts when the hosts defecate, now to be eaten by a land snail which in turn becomes the new host in the next asexual cycle of Dicrocoelium’s life. Within the snail the parasite develops to a new stage, the cercaria, which exits the snail enveloped in a mucus ball which is ingested by ants. Once inside an ant, the parasites bore through the stomach of the ant and one of them migrates to its brain and forms a thin-walled cyst known as the brain worm. The other ants form thick walled cysts. The brain worm changes the behaviour of the hapless ant, now the third of the ingeniously cunning Dicrocoelium’s unwilling hosts, causing it to spend more time on the tips of grass blades, where it is more likely to be swallowed by a grazing animal. The other cercariae, those that have formed the thick walled cysts which protect them as the herbivore chews its cud, migrate to the liver of the animal where the life cycle begins all over again. This is an interesting example of apparent altruism, as the brain worm is destroyed and has no opportunity to pass on its genes, but through its sacrificial action its conspecifics do, and in their book Unto Others Sobers and Wilson quote it as a possible example of such. But it seems to me to be easily accounted for by W.D. Hamilton’s explanation of apparent altruism in other species, especially social ones such as bees, termites and ants themselves. If the cercaria mated it would only pass on half its genes to offspring. In situations where it shares more than half its genes with its sisters it is genetically advantaged if more than one sister is enabled to pass on its genes by its actions. But I don’t know enough about trematodes to know whether this is the case.
I don’t think these wonders of intelligibility are credible except in a universe that is itself intelligent (aka God) but atheists of course don’t agree with that.