Splitting Planarians

Incomplete Thoughts on Theodicy (and Complete Ones on Certain Flatworms)

Planarians are hammer-headed flatworms with unusual regenerative abilities. If you were to take a planarian and cut it in two, both ends would re-grow into a complete planarian. (The same is falsely said of earthworms. If you cut an earthworm in two, the anterior portion will re-grow a tail and the posterior portion will die; moreover, the anterior portion will only re-grow a tail if it is more than 50% of a whole worm. If the anterior portion is less than half of the worm, both parts will die.) In fact, you could chop a planarian into over a dozen pieces any which way, and most or all of the pieces would grow into new worms. (Since those pieces cannot eat until they have become new worms, they cannot gain mass, so they actually break down and rearrange the tissues in their bodies to re-form the complete worm shape as a scale miniature.) Certain species of planarians reproduce exclusively in this way: an asexual planarian detaches its tail-end, which grows into a complete planarian in its own right. Other species of planarians reproduce both sexually and asexually. Planarians can regenerate in this way because they have adult stem cells called neoblasts distributed throughout their body.

Other animals have the ability to get chopped up and then grow into numerous organisms (starfish, arguably Voldemort's soul), but planarians aren't done being amazing. Because they have such regenerative abilities, planarians recover from injuries in ways that might seem bizarre to us. If a planarian receives a cut on its body, a tiny planarian will grow from the cut, eventually breaking off on its own. If you cut a planarian's head down the middle, the right half and the left half will each grow into a separate complete head; over time, the resulting flatworms will split down the middle and be two separate individuals, but for a while they share some length of their body.

I find planarians fascinating for a number of reasons. The first is that I am fascinated by conjoined twins and polycephaly; there is something very intimate about those who share (and/or compete for) a body, but I'm also interested in how having a body that departs from unquestioned cultural assumptions about bodies changes how people conduct their lives within that culture. The second is that planarians are an interesting case in how identities persist over time; if I were to cut a planarian straight down the middle, bisecting the bi-lobed cerebral ganglia sometimes considered its brain, how would the original planarian's identity persist into the two resulting individuals? Since the organism never dies I would say that either of the resulting planarians is the same organism as the original as much as I who write these words am the same organism as the person who planned to write them a few hours ago. But does that then make them the same organism as each other? That makes no sense. (Of course what we mean when we say "identity" is usually has more to do with memory and self-image than what organism we are, but I think it's still an interesting question with important ramifications.) And there are other reasons why I find planarians interesting.

When I'm in a poetic mood, though, I think about how, to a planarian, injury entails reproduction. Damage, if not lethal, is generative. I even wrote a sonnet about it once. This is lovely as a metaphor, of course, and I tried to make use of those metaphorical possibilities. But it recently occurred to me that a given planarian might not be so excited about reproductive regeneration. For us, as humans, our injuries hurt, scab over, heal, scar (sometimes). For them, as planarians, injuries produce other flatworms; at best these other flatworms are nutritional drains, competitors for the resources of one's own body, and at worst these other flatworms are volitional beings that try to swim in other directions than oneself, hampering movement and efficiency. That's maybe just the way of having children, but for most people children don't happen to you in the way that injuries do. If we were to ask a planarian how it felt about the worm budding out of its side or about the clone (that was once itself) with which it fights over its body, and if the planarian were capable of conscious and symbolic thought, it might answer that it would far prefer to reproduce sexually and to heal like other organisms do, that regeneration is great and all but this is just not worth it. Perhaps injuries are for the greater good: there are now two planarians to gripe about the situation rather than just one. All each of those planarians can see, though, is that the whole process has inconvenienced it. Then again, as far as I know planarians are incapable of conscious and symbolic thought, so perhaps it bothers them less than it might bother me.

The way I imagine a planarian would feel when asked to celebrate its injury is roughly how I feel about a lot of attempts at theodicy, or the secular versions of it, which claim that hardship makes you better. Today's reading from Romans has a version: "And not only that, but we also boast in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not disappoint us, because God’s love has been poured into our hearts through the Holy Spirit that has been given to us" (5:3-5). But religions are far from the only institutions which make these sorts of claims; exercise gurus say things like, "No pain, no gain," and military types say that "pain is just weakness leaving your body." It all makes me a bit resentful: from what I can tell, the kind of suffering I must endure does not produce any hope or strength. Why would it? I'm not saying that suffering is never ennobling--it probably is--but for goodness sake why must I suffer if there is some other way to be ennobled? Why, if I must grow, must I do it through injury?