Phyla: Acoelomorpha and Platyhelminthae
Conservation status: Not listed
Let Job bless with the Worm – the life of the Lord is in Humiliation, the Spirit also and the truth.
Christopher Smart
We are all worms,’ Winston Churchill said, ‘but I do believe that I am a glow-worm.’ The quip would earn him an F in biology (most glow-worms are a kind of fly or beetle) but an A for insight into how humans often feel. We know ourselves to be tiny specks in the universe but we can’t help feeling that we are really rather special. Or the loop of thought can run the other way: we’re amazing! . . . but there’s no escaping it, we are one with the worm. As the geneticist Steve Jones puts it, ‘every one of us, however eminent, is a ten-metre tube through which food flows, for most of the time, in one direction.’
Whichever way you put it, however, the fact is that in everyday life few of us spend much time thinking about worms. Beyond the rarefied worlds of evolutionary biology and parasitology, general attitudes to worms have changed little since people were writing bestiaries seven hundred years ago: there are various things out there which are wormy, the ones in the soil are good but most of the rest are to be avoided and . . . well, that’s it. This, I will argue, is a pity because it means people are missing out on many things that are remarkable and beautiful as well as repulsive and unsettling. When you get beyond the yuck factor, a whole world of delights – and frights – opens up. From arrow worms to spoon worms, and from peanut worms to penis worms, every human can benefit from contemplating the riot, the carnival, the salmagundi of worms.
In an account that was widely accepted for much of the twentieth century, complex animals – that is, creatures with organs such as hearts, guts and eyes, and made up of billions or even (as humans are) trillions of cells – were generally supposed to have evolved from single-celled organisms over a few million years from about 542 million years ago. This was the Cambrian explosion – the first great flowering from obscurity in what the evolutionary biologist Bill Hamilton called ‘the vast psychedelic drug enterprise of nature.’ But, as noted in Chapter 2 (Barrel Sponge), it is now clear that relatively simple multicellular forms had already existed for a hundred million years or more when the Cambrian began: the explosion had a long fuse, and during this earlier fizzling life experimented with various ways to big up. One was to be a sponge – an option which, as we have noted, continues to this day. Another was explored by the Ediacarians – a diverse group (or set of groups) ranging from the repeatedly branching (fractal) frond-like Charnia to the ribbed cushion-like Dickinsonia and the tri-radially symmetrical Tribrachidium, which resembled a triskelion mounted on a pizza. Truly, the Ediacaria, some of which grew to a metre (three feet) or more across, evolved beyond psychedelia. As Italo Calvino’s hero Qfwfq says in Cosmicomics, ‘When you’re young, all evolution lies before you . . . If you compare yourself with the limitations that came afterwards, if you think how one form excludes other forms, of the monotonous routine where you finally feel trapped, well, I don’t mind saying, life was beautiful in those days.’
Alas, many paleobiologists now think that, for all their glory, the Ediacarians left few or no descendants in the Cambrian. For whatever reason they were superseded by various animal phyla including our own, the chordates, and many that were wormy. What exactly these were all descended from is not known, but one possibility is suggested by an enigmatic trace in 600-million-year-old rock which some interpret as the fossil of a creature they call Vernanimalcula or ‘spring animal’. This (probably) worm-like thing, if it was an animal, was no thicker than a human hair. But it was only with the passing of the Ediacarians that the descendants of Vernanimalcula, or whatever it was that gave rise to all the complex animals we know today, came into their own.
The great diversification of life in the Cambrian most likely resulted from a combination of factors. Rising levels of oxygen in the ocean, which allowed animals to get bigger, probably played an important role to begin with. The evolution of eyes may have then driven an arms race between predators and prey. And the emergence of new, more efficient means of predation and foraging – ‘terrors with teeth’ with a through-gut (an early, small version of the ‘tube’ which Steve Jones reminds us we all are) connected to an anus, which could process what they ate with more efficiency than any previous animal – may have been even more important than the evolution of eyes. But whatever the cause, the result was the spectacular radiation into virtually all the forms we see in the world today. Whether or not most of philosophy is a footnote to Plato, much of life since the Cambrian has been little more than a footnote to the step-change achieved by these early creatures in their ability to eat, digest and excrete the world around them.
Some of the earliest creatures to acquire sharp teeth, efficient guts and anuses were worm-like. The first ‘terror with teeth’ may have been a kind of arrow worm or Chaetognath. One of the commonest fossils found in some Cambrian shales, observes Martin Brasier, resembles a novelty condom with an organ inside. Paraselkirkia had a bulbous head ornamented with a spiky helmet. Its head was attached to a long wrinkled body and the whole was protected by what looks to have been a rubbery organic sheath. Paraselkirkia was a kind of Priapulid, or penis worm – a phylum of animals that lives in mud and eats it too. Another creature that appears to have been quite widespread is Hallucigenia. This creature met with fame after its discovery in 1977 because of the bizarre appearance it was thought to have had and which had inspired its name. Hallucigenia, it was believed, had long rigid spikes instead of feet on its underside and must have moved around as if on multiple stilts, with little tentacles waving on its back. Later analysis showed, however, that this animal was being imagined upside down: the tentacles were small legs and the spikes were protection on its back, rather as we see on some caterpillars today. Hallucigenia, it turns out, may have been a kind of Onychophor, or Velvet worm. Others in this remarkable phylum including Microdictyon evolved huge false compound eyes – mimicry to warn off would-be predators.
Velvet worms may have been quite common in the Cambrian. Indeed, this may have been their golden age. Nowadays they are mostly found under rocks or in rotting trees in remote parts of the southern hemisphere, and until recently they have been neglected or had something of a bad press. In his magisterial Life: An Unauthorised Biography (1997), Richard Fortey called them ‘primitive’. Today, however, Velvet worms are recognized as remarkable beasts, and many biologists, including Fortey, are more sensitive of their virtues. They are highly social and live in close groups with clearly established hierarchies. They cooperate to hunt and tend to be hostile to other groups. Their mating rituals – the male has a penis-like organ on his head, which he inserts into the female – and their virtuoso ability to squirt sticky slime in the face of enemies and prey have made them popular as pets. Even more strikingly, modern varieties appear to be very similar to fossils as much as 540 million years old.
Velvet worms also have teeth of a kind. Deep within the oral cavity lie sharp, crescent-shaped mandibles that resemble the claws of their feet but are strongly hardened. The mandibles are divided into internal and external sets and each is covered with fine toothlets. They move backward and forward to tear apart prey.
For all the success of Velvet worms in the Cambrian, however, other creatures evolved even more formidable features for attack and defence. Arthropods, which share a common ancestor with Velvet worms, developed armour and jointed limbs which gave them a huge advantage over their softer and squidgier cousins. Early Chordates, deriving from a common ancestor with Ragworms, developed comparatively sophisticated brains, and in time skulls to protect them. The result may have looked a little like a hagfish – a kind of halfway house between worm and fish (or perhaps a lancelet). Later, still other creatures evolved from these to have spines and other bones on which to anchor stronger muscles. These, the first vertebrates, were the earliest true fish: jawless Ostracoderms in the late Cambrian and the Ordovician and then, from the early Devonian, the Placoderms – large animals with powerful jaws mounted in their heavily armoured heads. Placoderm armour is made from exactly the same material as teeth.
But even with the rise of larger animals – the vertebrates (such as fish), molluscs (snails and cephalopods), arthropods (crustaceans and insects) and echinoderms (starfish) – several phyla of worm-like creatures continued to evolve and proliferate. Many indeed, made their homes as parasites in these other animals rather as their ancestors had made their homes in the mud and silt of the Cambrian seabed. In addition to those already mentioned, Jaw worms, Acorn (or tongue) worms, Horsehair worms, Ribbon worms (also known as Proboscis worms), Horseshoe worms and Peanut worms all flourished and continue to do so today. (These are named for their appearance, not where they live: many are residents of the deep sea.) Many are microscopic (and parasitic), but a few are huge. Bootlace worms, which are a kind of Ribbon worm, can grow to thirty metres (ninety-eight ft) long. With an evertible proboscis – a bit like an elephant’s trunk that turns inside out – they scavenge the seabed for small sponges, jellyfish, anemones and fish. As one of the longest animals in the world they sound like a terrifying predator until you learn that their bodies are no thicker than a pencil. Ribbon worms are no mighty dragons; happening upon a typical species on the seabed you might think you had run into a pile of spilled intestines.
But there are three phyla of worms that, in terms of diversity and abundance, tower over all the rest: the Roundworms (nematodes), the Annelids and the Flatworms. And before coming to the third of these, a few words of celebration of the first two phyla are in order.
Roundworms may be the most diverse and numerous of all the wormy phyla. Many are parasites, and so it’s easy – perhaps too easy – to pass over them with a shudder. But others achieve remarkable feats without feet: the splendidly named H. mephisto was recently discovered where multicellular life was thought impossible, nearly 3,000 metres (9,000 ft) beneath the surface of the Earth in a goldmine. And at least one species is likely to add greatly to the sum of human happiness: C. elegans – a transparent and easily reproducible being (it is a self-fertilizing hermaphrodite that matures to a 1 mm-long adult in three and half days to produce around 300 offspring, a few of which are male) – has been a favourite model organism in labs for many years, used in research into the fundamentals of gene expression, development and other processes seen across the animal kingdom. In 1998 it became the first creature to have its genome (one of the smallest of any animal) sequenced. Its simple nervous system was the first to be fully mapped; it works fine with only about 300 neurons. C. elegans is truly elegant for doing so much with so little. At least four Nobel Prizes in physiology or medicine have been awarded since 2000 for research that has depended on this tiny worm.
Annelids, the segmented worms, are also a large and tremendously diverse group, ranging from the most familiar of all worms – garden earthworms and seaside lugworms – to some of the most bizarre-looking species yet discovered, such as the two-metre-long tube worms and the smaller Pompeii worms which bathe in scalding temperatures on volcanic vents in the deep ocean, and the bristly Christmas tree worms that achieved fame in impossibly large but otherwise largely accurate translation to the planet Pandora in the film Avatar. Earthworms were the first worms to be the subject of sustained and serious scientific interest, when Charles Darwin undertook to study their behaviour and effect on their environment in the garden of his home in Kent. Darwin appreciated, in a way that almost no one had done before, that it was earthworms which made the earth. He also observed, to his considerable surprise, that they demonstrated significant powers of reason, making intelligent decisions as to what shape of leaf to use to block their holes, and how.
And so to the flatworm. Broadly speaking, this is an animal with no body cavity in which to house a heart, lungs or gut: its insides have no inside, and as a result it is restricted to flat shapes that allow oxygen and nutrients to pass through by diffusion. But ‘flatworm’ is a generic name for many thousands of species that fall into at least three groups and the differences between them are as great as the similarities. This is one of the reasons I chose them for this bestiary. They are a reminder that big differences and subtle details are often hidden by language and thinking that is too blunt. (Such, at least, is the case with me: until I started researching I had little idea what flatworms were, still less how they differed.) Some flatworms have evolved life cycles as gruesome as any imaginable. Others are among the most brightly coloured members of the animal kingdom. Still others engage in what may be the most startling sexual practices on the planet. With their dark, light and bizarre sides, these various organisms with a misleading singular label make a good talisman for meditation on life and death.
Flatworms are actually creatures from two different phyla. They can also be divided into three groups according to lifestyle. One group, comprising more than half the species in one of the two phyla, the platyhelminths, consists of parasites. The other two groups are free-living. One of these, the Turbellaria, also consists of platyhelminths. But the other, the Acoelomorpha (or Acoels), is probably no more closely related to platyhelminths than it is to us. Typically the width of a peppercorn and as flat as a pancake, Acoels have no brain or ganglia but a network of nerves beneath the skin that is slightly more concentrated towards the front end. They have a simple organ for balance called a statocyst that works a little like the vestibular system in the human inner ear, and some species have simple eyespots for detecting the presence or absence of light. Unlike, say, the eight-eyed box jellyfish, which looks the same in every direction, Acoels would probably pass the threshold set by Thomas Browne for non-mythical animals – that they have a front end and a back end, a left and a right. But at least one species has virtually given up being animals. In youth, Convoluta roscoffensis swallows green algae with all the enthusiasm of teenagers on alcopops and never bothers to feed again, relying entirely on the photosynthesizing algae to nourish it. On its native shores, Convoluta rises from the damp sands of the intertidal zone as soon as the tide has ebbed, and the sand becomes blotched with large green patches of ‘slime’ composed of thousands of worms, which photosynthesize in the sunlight until the flood returns and they disappear beneath the sand again. Remarkably, a colony of these worms in an aquarium or laboratory tank will continue this behaviour, seeking sunlight twice each day. Rachel Carson writes: ‘Without a brain, or what we would call a memory, or even any very clear perception, Convoluta continues to live out its life in this alien place, remembering, in every fibre of its small green body, the tidal rhythm of the distant sea.’
Several species among the Turbellaria (or Planarians, as free-living platyhelminths are also known) have a goofy pair of little eyes, making them perhaps the cutest of all worms. Like other platyhelminths (that is, all flatworms apart from Acoels), their lack of an inside, or coelum, is ‘secondarily derived,’ which means they are descended from organisms that had one but have abandoned it along the evolutionary way as so much unnecessary baggage, rather as humans have for the most part abandoned fur and tails. Some Turbellaria have adopted the dazzling and diverse colour patterns of nudibranchs, sophisticated and agile molluscs to which they are not related. Nudibranchs are often poisonous, so mimicking them has clear advantages. And when it comes to combining two of humanity’s favourite activities – sex and combat – nothing beats Turbellaria. These animals, which are hermaphrodites, engage in spectacular penis fencing, using two phalluses mounted on their chests as weapons with which they attempt to pierce and impregnate each other.
The other large group of platyhelminths (and more than half of the thousands of the known species of this phylum) are parasites – flukes, tapeworms and other lovelies. Some do great damage to humans and other animals. Trematodes are responsible for schistosomiasis, the second most devastating human disease caused by parasites after malaria (which is caused by protists of the genus Plasmodium). When larvae of Taenia solium, the pork tapeworm, penetrate the human central nervous system they cause neurocysticercosis, a particularly nasty form of epilepsy. Tapeworms that live in the human gut may look scary but they are benign by comparison.
Few things are more intimately horrible than a tapeworm – creatures which take up residence in our guts, our livers, even our brains and gorge on our lifeblood. (When, a few years ago, a senior editor on the Wall Street Journal was looking for a really nasty epithet for Google, he called it a tapeworm, following a long-established practice of equating things we truly hate and fear with parasites.)
Our fear and loathing for parasites is obviously adaptive. But this fear can itself mutate into a psychopathology – a phenomenon well documented in different times and cultures. Notably, there is a condition called delusional parasitosis, in which an individual hallucinates parasites crawling out of every orifice. Anxiety and fear can also be captured by others and turned to political ends. The Nazis cultivated anti-Semitism, for example, by associating Jews and other out-groups with parasites.
Summoning any kind of enthusiasm for parasitic flatworms (or any sort of parasite) is, then, hard to do. But if we cannot be enthusiastic, can we not at least learn to appreciate them – where ‘to appreciate’ means ‘to better understand their importance and impact’ rather than ‘to like’? For one thing, tapeworms have been our constant companions. Homo ergaster, the earliest member of the human genus, Adam had’em. Some of the pathogenic bacteria in our gut may date vastly further back as they are shared with organisms living at the bottom of the deep sea.
‘Everything that lives is holy/Life delights in life’, wrote William Blake. But the truth is that life often delights in the death of other life and – even more disturbingly – eating other things while they are still alive is the most popular lifestyle on Earth. Virtually every multicellular animal that lives is loaded with parasites. Expressed in terms of biomass – sheer weight – parasites actually outweigh large predators, from sharks to lions, in some ecosystems, sometimes by as much as twenty times. This reality may seem horrific at first, particularly when one considers the effect of some parasites: hollowed-out or deformed bodies, chemical castration, brainwashing and bizarre behaviour that makes an infected animal more vulnerable to being eaten by others. The whole world can start to look diseased, like the vision of death in life experienced by Coleridge’s Ancient Mariner before his redemption. Ray Lankester, an influential zoologist of the generation after Darwin, believed parasites were a contemptible outcome of evolutionary degeneration (in which an organism becomes dependent upon others) – a fate which he believed was awaiting Western civilization too.
From the larger, evolutionary perspective, the view is rather different. Parasites are frequently harmless and may even be beneficial to a species and the ecosystem of which they are part. Their presence in large numbers can actually be a sign of health. And some of them – contrary to Lankester’s prejudice – are enormously sophisticated. The parasite that causes toxoplasmosis, which is present in as many as a third of all humans, ‘knows’ how to access certain specific circuits in the amygdala of its target host, which is actually the rat, so that it will lose its fear of the odour of its predators. In some respects ‘toxo’ has a better understanding of how mammal brains work than neuroscientists do. Even more significantly, if the hypothesis is true, parasites may have played a role in driving the evolution and persistence of sex in the animal world: only by giving birth to offspring that are not genetically identical are so many species able to find new ways of combatting the endless assaults of parasites.
For all that, in most human experience parasites are one of the many heralds of death: the reality – or end to reality – often said to be humanity’s greatest puzzle and challenge. But just as we can broaden our knowledge of flatworms beyond the nasty tapeworms so, maybe, we can take a wider view of death.
A drive to overcome death has dominated much of our behaviour for as long as we have been human. Other animals may share our hair-trigger awareness of dangers but none, it seems, has our ability or our tendency to imagine the opposite of vivacity so vividly and relentlessly. This ‘tragedy of cognition’, in a phrase coined by the anthropologist Scott Atran, has been part of us since, perhaps, around 500,000 years ago when the beginnings of language began to enhance our awareness of absent others. Death has always been a looming presence, a lurking, silent interlocutor behind a bewildering variety of masks, with whom we have an intermittent but unending dialogue in our heads.
Perhaps we need to entertain different thoughts about death almost as if we were replaying the evolution of the most flamboyant marine flatworms, and trying on different colours as we go. Who knows which we will find most compelling, or which colours we will be wearing when oblivion finally unmoors us? In the interim should we live as if death is nothing, or keep it constantly in mind? Can we find one attitude, or a combination of attitudes, that will be vaguely adequate in the face of reality? Will even our best shot be a kind of anasognosia, a complex form of denial with many layers to it? In a surviving fragment of Niobe, Aeschylus writes:
Alone of gods, Death has no use for gifts
Libations don’t help you, nor does sacrifice
He has no altar, and hears no hymns;
He is not amenable to persuasion.
Even for those who consider themselves eminently rational and for whom death holds no mysteries, there are still factors beyond rational control to contend with. One’s own death may be fairly easy to accept, for example, but the death of a beloved (or one’s greatest hope) can be almost unbearable. After the death of his daughter Tullia in childbirth, Cicero looked to the doctrine of Stoicism, which holds that one should practice indifference to things one cannot control. But he found it wholly inadequate to the emotional realities. ‘It is not within our power to forget or gloss over circumstances which we believe to be evil,’ he wrote. ‘They tear at us, buffet us, goad us, scorch us, stifle us — and you [Stoics] tell us to forget about them?’ Montaigne found his own first close brush with death relatively untroubling but was devastated by the death of his friend Etienne de la Boétie.
The second law of thermodynamics dictates that every physical system tends towards maximum disorder. Life is just a system, and even life must eventually end. Eternity does not exist. Everything will become very dark and very cold – that is, almost as bad as England in winter. Some of the least deceived minds of the late nineteenth century found this harsh truth at the heart of physical law almost too tough to accept. (In this respect, physics proved to be the opposite of something Marx had said about religion – that it was ‘the heart of a heartless world’.)
Writing early in the twentieth century Bertrand Russell, a stubborn Englishman if ever there was one, advocated heroic defiance:
. . . all the labours of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man’s achievement must inevitably be buried beneath the debris of a universe in ruins – all these things, if not quite beyond dispute, are yet so nearly certain, that no philosophy which rejects them can hope to stand. Only within the scaffolding of these truths, only on the firm foundation of unyielding despair can the soul’s habitation be safely built.
For Russell this foundation was enough for a life well lived. Fifty years later it still supported him as a human dynamo behind what became known as the Russell–Einstein Manifesto, which challenged the omnicidal policies embraced by the superpowers during the Cold War and stands as one of the great statements of humanism. It is a good example of asserting the value of the ‘little’ Earth we have actually in front of us – the Pale Blue Dot, as Carl Sagan was later to call it – rather than appealing to some invisible transcendent.
Scientific advances during Russell’s lifetime have cast new light on the nature of reality such that the austerity of the Second Law is a little more bearable. For one thing, we now think the universe has vastly longer to go than people believed at the end of the nineteenth century: several billion years at the least rather than a few million. For another, advances in the biological sciences allow for ever-increasing appreciation for the nature of life – not least the extraordinary conjuring trick it pulls off by drawing order from the stream of increasing disorder in the universe around it. This trick affords life astounding possibilities for a future that is, if not indefinite, at least almost unimaginably long. As Russell himself said, in what for him was an almost mystical statement, ‘the world is full of magical things patiently waiting for our wits to grow sharper.’
After an unexpected brush with death in what he had thought should have been only the middle of his life, the earth systems scientist Tyler Volk set out to understand his own mortality and the world’s. His answer, in the end, is a simple one. At a material level, life cannot exist without death: recycling of organic matter in the biosphere makes it about two hundred times more productive than it would otherwise be. Our bodies, too, must become tilth. At the emotional and spiritual level, the key is acceptance. In Blake’s phrase, ‘He who kisses the joy as it flies, lives in eternity’s sun rise.’
As we prepare to return to the material stuff of Darwin’s ‘one long argument’, the process of dying can be genial, as David Hume showed in the humour and insight he brought to his last days. And even the state of death itself can – if the writer and fanatical gardener Karel Čapek is any guide – be looked forward to with something like gusto: ‘After his death the gardener does not become a butterfly, intoxicated by the perfumes of flowers, but a garden worm, tasting all the dark, nitrogenous and spicy delights of the soil.’
Whether or not one believes in life after death, it is the case that an entire Planarian flatworm can be regrown from a single cell taken from the body of an adult. Evidence enough that there are miracles in life.
