The Ancestor's Tale Part 16

(18651939) 5 I have discussed this at length in I have discussed this at length in Climbing Mount Improbable Climbing Mount Improbable, in a chapter called 'The Fortyfold Path to Enlightenment', and I return to it at the end of this book.

6 Following the theoretical idea known as the Baldwin Effect. Superficially, it sounds like Lamarckian evolution and the inheritance of acquired characteristics. Not so. Learning doesn't imprint itself into the genes. Instead, natural selection favours genetic propensities to learn certain things. After generations of such selection, evolved descendants learn so fast that the behaviour has become 'instinctive'. Following the theoretical idea known as the Baldwin Effect. Superficially, it sounds like Lamarckian evolution and the inheritance of acquired characteristics. Not so. Learning doesn't imprint itself into the genes. Instead, natural selection favours genetic propensities to learn certain things. After generations of such selection, evolved descendants learn so fast that the behaviour has become 'instinctive'.

7 Bug means something precise, not just anything small. A bug is an insect of the order Hemiptera. Bug means something precise, not just anything small. A bug is an insect of the order Hemiptera.

8 So has the nudibranch mollusc (sea slug) So has the nudibranch mollusc (sea slug) Glaucus atlanticus Glaucus atlanticus. This beautiful creature floats upside down, feeding on Portuguese men-of-war, and it is 'reverse countershaded', just like the catfish.

9 Although I have to admit that the habits of the upside-down catfish have long been known. It is portrayed in its customary position in Ancient Egyptian wall paintings and engravings. Although I have to admit that the habits of the upside-down catfish have long been known. It is portrayed in its customary position in Ancient Egyptian wall paintings and engravings.

10 You might ask how an experimenter could force a catfish to turn over against its natural preference, and I do not know. But, to add just one tiny vignette, I do know how to make a brine shrimp swim, like any normal crustacean, with its zoologically dorsal side uppermost. Just s.h.i.+ne an artificial light on them from below, and they will instantly turn over. Evidently brine shrimps use light as their cue to decide which way is up. I don't know if the catfish use the same cue. They could equally well use gravity. You might ask how an experimenter could force a catfish to turn over against its natural preference, and I do not know. But, to add just one tiny vignette, I do know how to make a brine shrimp swim, like any normal crustacean, with its zoologically dorsal side uppermost. Just s.h.i.+ne an artificial light on them from below, and they will instantly turn over. Evidently brine shrimps use light as their cue to decide which way is up. I don't know if the catfish use the same cue. They could equally well use gravity.

11 Stridulation is how gra.s.shoppers, and crickets, make sound. Gra.s.shoppers sc.r.a.pe their legs against their wing covers. Crickets sc.r.a.pe the two wing covers against each other. They sound similar, but gra.s.shoppers are generally more buzzy, crickets more musical. Of one nocturnal tree cricket it has been said that if moonlight could be heard, that is how it would sound. Cicadas are quite different. As if buckling a tin lid, they buckle part of the thorax wall, repeatedly and fast, so it sounds like a continuous buzz, usually extremely loud and sometimes patterned in very complex ways, characteristic of the species. Stridulation is how gra.s.shoppers, and crickets, make sound. Gra.s.shoppers sc.r.a.pe their legs against their wing covers. Crickets sc.r.a.pe the two wing covers against each other. They sound similar, but gra.s.shoppers are generally more buzzy, crickets more musical. Of one nocturnal tree cricket it has been said that if moonlight could be heard, that is how it would sound. Cicadas are quite different. As if buckling a tin lid, they buckle part of the thorax wall, repeatedly and fast, so it sounds like a continuous buzz, usually extremely loud and sometimes patterned in very complex ways, characteristic of the species.

12 As an aside, leopards don't either. But black 'panthers', once thought to be a separate species, differ from spotted leopards at a single genetic locus. As an aside, leopards don't either. But black 'panthers', once thought to be a separate species, differ from spotted leopards at a single genetic locus.

13 As it happens, Lewontin himself was one of the first biologists to use information theory, and indeed he did so in his paper on race, but for a different purpose. He used it as a convenient statistic for measuring diversity. As it happens, Lewontin himself was one of the first biologists to use information theory, and indeed he did so in his paper on race, but for a different purpose. He used it as a convenient statistic for measuring diversity.

14 Sir Roger Bannister got into terrible hot water, for no good reason that I could discern except people's hair-trigger sensitivities on matters of race, when he said something similar a few years ago. Sir Roger Bannister got into terrible hot water, for no good reason that I could discern except people's hair-trigger sensitivities on matters of race, when he said something similar a few years ago.

15 Imprinting is the process, often said to have been discovered by Konrad Lorenz, whereby young animals, for instance goslings, take a kind of mental photograph of an object they see during a critical period early in life, and which they follow while young. Usually it will be a parent, but it could be Konrad Lorenz's boots. Later in life, the 'mental photograph' influences choice of mate; this usually means a member of their own species, but they might try to mate with Lorenz's boots. The gosling story isn't as simple as that, but the a.n.a.logy to the insect case should be clear. Imprinting is the process, often said to have been discovered by Konrad Lorenz, whereby young animals, for instance goslings, take a kind of mental photograph of an object they see during a critical period early in life, and which they follow while young. Usually it will be a parent, but it could be Konrad Lorenz's boots. Later in life, the 'mental photograph' influences choice of mate; this usually means a member of their own species, but they might try to mate with Lorenz's boots. The gosling story isn't as simple as that, but the a.n.a.logy to the insect case should be clear.

16 A potential problem, which would need sorting out if the idea were to be pursued, is that the theory of mathematical genetics suggests, for geographical separation and by implication this cultural hypothesis too, that the separation has to be pretty complete for genetic differentiation to be maintained. A potential problem, which would need sorting out if the idea were to be pursued, is that the theory of mathematical genetics suggests, for geographical separation and by implication this cultural hypothesis too, that the separation has to be pretty complete for genetic differentiation to be maintained.

17 This favourite a.n.a.logy was first used by my friend Sir Patrick Bateson, a relative of Sir William, as it happens. This favourite a.n.a.logy was first used by my friend Sir Patrick Bateson, a relative of Sir William, as it happens.

18 Some other insects, such as c.o.c.kroaches and beetles, fly with only T3 wings, having modified the T2 wings into hardened protective wing cases called elytra. Crickets and gra.s.shoppers, as we have heard, further modified the elytra as sound-producing organs. Some other insects, such as c.o.c.kroaches and beetles, fly with only T3 wings, having modified the T2 wings into hardened protective wing cases called elytra. Crickets and gra.s.shoppers, as we have heard, further modified the elytra as sound-producing organs.

19 To be scrupulously correct, in nearly 300 years of scientific investigation, there has been a single report of a male bdelloid rotifer made by the Danish zoologist C. Wesenberg-Lund (18661955). 'With great hesitation I venture to remark, that twice I saw among the thousands of Philodinidae ( To be scrupulously correct, in nearly 300 years of scientific investigation, there has been a single report of a male bdelloid rotifer made by the Danish zoologist C. Wesenberg-Lund (18661955). 'With great hesitation I venture to remark, that twice I saw among the thousands of Philodinidae (Rotifer vulgaris) a little creature, unquestionably a male rotifer ... but both times I failed to get it isolated. It moves round and between the numerous females with extreme rapidity' (understandably, no doubt). Even before the strong evidence of Mark Welch and Meselson (see page 440) zoologists were not inclined to take Wesenberg-Lund's never-repeated observation as sufficient proof of the existence of male bdelloids.

20 Meiosis is the special form of cell division that halves the number of chromosomes in order to make s.e.x cells. Mitosis is the ordinary form of cell division used for making body cells, which duplicates all the chromosomes of a cell. Meiosis is the special form of cell division that halves the number of chromosomes in order to make s.e.x cells. Mitosis is the ordinary form of cell division used for making body cells, which duplicates all the chromosomes of a cell.

21 People sometimes confusingly say gene pool when they mean genome. The genome is the set of genes within one individual. The gene pool is the set of all genes in all the genomes of a s.e.xually breeding population. People sometimes confusingly say gene pool when they mean genome. The genome is the set of genes within one individual. The gene pool is the set of all genes in all the genomes of a s.e.xually breeding population.

22 The great scientist J. B. S. Haldane offered a completely different Barnacle's Tale, a parable in which philosophical barnacles contemplate their world. Reality, they conclude, is everything they can reach with their filtering arms. They are dimly aware of 'visions', but doubt their physical reality because barnacles on different parts of the rock disagree as to their distance and shape. This clever allegory on the limitations of human thought and the growth of religious superst.i.tion is Haldane's tale, not mine, and I shall merely recommend it and pa.s.s on. It is in the eponymous essay of The great scientist J. B. S. Haldane offered a completely different Barnacle's Tale, a parable in which philosophical barnacles contemplate their world. Reality, they conclude, is everything they can reach with their filtering arms. They are dimly aware of 'visions', but doubt their physical reality because barnacles on different parts of the rock disagree as to their distance and shape. This clever allegory on the limitations of human thought and the growth of religious superst.i.tion is Haldane's tale, not mine, and I shall merely recommend it and pa.s.s on. It is in the eponymous essay of Possible Worlds Possible Worlds.

23 I went to town on such cases of parasites subtly manipulating the intimate physiology of their hosts in the parasite chapter of I went to town on such cases of parasites subtly manipulating the intimate physiology of their hosts in the parasite chapter of The Extended Phenotype The Extended Phenotype.

24 Bertrand Russell, of course. Bertrand Russell, of course.

25 A fourth site, Orsten ('stink stone') in Sweden, preserves soft bodies in a different way. A fourth site, Orsten ('stink stone') in Sweden, preserves soft bodies in a different way.

26 When the molecular clock was first proposed, by Emile Zuckerkandl and the great Linus Pauling, this was the only method available. When the molecular clock was first proposed, by Emile Zuckerkandl and the great Linus Pauling, this was the only method available.

27 The DNA code being 'degenerate', any one amino acid can be specified by more than one 'synonymous' mutation. A mutational change resulting in an exact synonym makes no difference at all to the final outcome. The DNA code being 'degenerate', any one amino acid can be specified by more than one 'synonymous' mutation. A mutational change resulting in an exact synonym makes no difference at all to the final outcome.

Rendezvous 27 ACOELOMORPH FLATWORMS.

When we were talking about the protostomes, descendants of Concestor 26, I grouped the flatworms, Platyhelminthes, firmly within them. But now we have an interesting little complication. Recent evidence quite strongly suggests that the Platyhelminthes are a fiction. I'm not saying flatworms themselves don't exist, of course. But they are a heterogeneous collection of worms who should not be united under one name. Most of them are true protostomes and we met them at Rendezvous 26 Rendezvous 26, but a few of them are quite separate and don't join us until here at Rendezvous 27 Rendezvous 27. This we are dating at 630 million years ago, although out in these remote reaches of geological time these datings become more and more uncertain.

Six hundred and thirty million years is quite a lot older than the 590-million-year date we adopted for Rendezvous 26 Rendezvous 26. Perhaps the long gap can be explained by the 's...o...b..ll Earth' episode which, according to one imaginative theory, preceded the Cambrian. The idea is that, for reasons that are obscure but may have to do with the fas.h.i.+onable and possibly overrated mathematical theory of chaos, the entire Earth was gripped by a global ice age from about 620 million years ago to about 590 million years, rather neatly filling the large gap between Rendezvous 27 Rendezvous 27 and and 26 26. There was plenty of glaciation. But whether or not the glaciations engulfed the entire planet is a contentious question, and one that I shall pa.s.s over.

What all flatworms have in common, apart from their eponymous flatness, is that they lack an a.n.u.s and they lack a coelom. The coelom of a typical animal, such as you or me or an earthworm, is the body cavity. This doesn't mean the gut: the gut, though a cavity, is topologically part of the outside world, the body being a topological doughnut, the hole in the middle of the ring being the mouth, the a.n.u.s and the gut that connects them. The coelom, by contrast, is the cavity within the body in which the intestines, the lungs, heart, kidney and so on all sit. Platyhelminths don't have a coelom. Instead of a body cavity in which the guts slop about, flatworm guts and other internal organs are embedded in solid tissue called parenchyma. This may seem a trivial distinction, but the coelom is embryologically defined and lies deep in the collective unconscious of zoologists.

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Acoelomorph flatworms join. The vast majority of bilaterally symmetrical animals are protostomes or deuterostomes. However, recent molecular data hive off a pair of flatworm groups as neither protostomes nor deuterostomes but earlier branching lineages. These are the cla.s.ses Acoela (about 320 described species) and Nemertodermatida (ten described species), together known as the acoelomorph flatworms. This is likely to be quickly accepted by taxonomists. Current evidence hints that the Acoela and the Nemertodermatida are sister groups, as shown here. The vast majority of bilaterally symmetrical animals are protostomes or deuterostomes. However, recent molecular data hive off a pair of flatworm groups as neither protostomes nor deuterostomes but earlier branching lineages. These are the cla.s.ses Acoela (about 320 described species) and Nemertodermatida (ten described species), together known as the acoelomorph flatworms. This is likely to be quickly accepted by taxonomists. Current evidence hints that the Acoela and the Nemertodermatida are sister groups, as shown here.

Image: unknown acoel flatworms on bubble coral. unknown acoel flatworms on bubble coral.

Lacking an a.n.u.s, how do flatworms expel wastes? Through the mouth if there is nowhere else. The gut may be a simple sac or, in larger flatworms, it branches into a complicated system of blind alleys, like the air tubes in our lungs. Our lungs, too, could theoretically have had an 'a.n.u.s' a separate hole for the air to leave by, with its waste carbon dioxide. Fish sort of do the equivalent, for their respiratory stream of water enters by one hole, the mouth, and leaves by others, the gill apertures. But our lungs are tidal, and so is the digestive system of flatworms. Flatworms lack lungs or gills and breathe through their skins. They also lack a system of circulating blood, so their branched gut presumably serves to transport nutriment to all parts of the body. In a few turbellarians, especially those with an exceptionally complex branched gut, an a.n.u.s (or lots of a.n.u.ses) has been reinvented after a long absence.

Because flatworms lack a coelom and mostly lack an a.n.u.s, they have always been regarded as primitive the most primitive of the bilaterally symmetrical animals. It was always a.s.sumed that the ancestor of all deuterostomes and protostomes was probably something like a flatworm. But now, as I began by saying, molecular evidence suggests that there are two unconnected kinds of flatworms, and only one of these two kinds is genuinely primitive. The genuinely primitive kind are the Acoela and the Nemertodermatida. The Acoela are named for their lack of coelom which, for them and the Nemertodermatida but not the Platyhelminthes proper, is a primitive lack. The main group of flatworms proper, the flukes, tapeworms and turbellarians, are now thought to have lost their a.n.u.s and their coelom secondarily. They pa.s.sed through a stage of being more like normal Lophotrochozoa, then reverted to being like their earlier ancestors again, sans sans a.n.u.s and a.n.u.s and sans sans coelom. They joined our pilgrimage at coelom. They joined our pilgrimage at Rendezvous 26 Rendezvous 26, along with all the rest of the protostomes. I won't go into the detailed evidence, but will accept the conclusion that the Acoela and the Nemertodermatida are different and join us as a tiny incoming stream here at Rendezvous 27 Rendezvous 27.

At this point I should describe these tiny worms that are joining us but, though I hate to say it, at least by comparison with most of the wonders we have seen, there is not a lot to describe. They live in the sea and they not only lack a coelom but lack a proper gut too a situation that's viable only in animals that are very small, which they are.

Some of them supplement their diet by giving house room to plants, and hence benefiting indirectly from their photosynthesis. Members of the genus Waminoa Waminoa have symbiotic dinoflagellates (unicellular algae) and live off their photosynthesis. Another acoel, have symbiotic dinoflagellates (unicellular algae) and live off their photosynthesis. Another acoel, Convoluta Convoluta, has a similar relations.h.i.+p with a single-celled green alga, Tetraselmis convolutae Tetraselmis convolutae. Symbiotic algae presumably make it possible for these little worms to be less little. The worms seem to take steps to make life easier for their algae, and hence themselves, crowding at the surface to give them as much light as possible. Professor Peter Holland writes to me that Convoluta roscoffensis Convoluta roscoffensis ... are amazing animals to see in their natural habitat. They appear as a green 'slime' on certain beaches in Brittany, the slime really being thousands of acoels plus their endosymbiotic algae. And as you creep up on the 'slime' it hides! (by disappearing into the sand). Very strange to see.

The Acoela are still with us, and therefore must be treated as modern animals, but their form and simplicity suggest that they might not be greatly changed since the time of Concestor 27. Modern acoel worms might be a reasonable approximation to the ancestor of all bilaterally symmetrical animals.

Our gathered pilgrims now include all the phyla recognised as Bilateria, which means the great bulk of the animal kingdom. The name refers to their bilateral symmetry, and is intended to exclude the two main radially symmetrical phyla, grouped together as the Radiata, who are now about to join the pilgrimage: the Cnidaria (sea anemones, corals, jellyfish, etc.) and the Ctenophora (comb jellies). Unfortunately for this simple terminology, starfish and their kin, which zoologists are sure are descended from Bilateria, are also radially symmetrical, at least in the adult phase. Echinoderms are a.s.sumed to have become secondarily radial when they took to a bottom-living existence. They have bilaterally symmetrical larvae, and are not closely related to the 'truly' radiate animals such as jellyfish. Reflexively, not all the cnidarians (sea anemones and their kind) are (quite) radially symmetrical, and some zoologists think they too had bilaterally symmetrical ancestors.

All in all, Bilateria is an unfortunate name by which to unite the descendants of Concestor 27 and separate them from those pilgrims still to join. Another possible criterion is 'triploblasty' (three layers of cells) versus 'diploblasty' (two). At a crucial stage in their embryology, cnidarians and ctenoph.o.r.es build their bodies out of two main layers of cells ('ectoderm' and 'endoderm'), the Bilateria out of three (they add 'mesoderm' in the middle). Even this is open to dispute, however. Some zoologists believe 'Radiata' also have mesodermal cells. I think the sensible thing is not to worry about whether Bilateria and Radiata are really good words to use, nor diploblastic and triploblastic, but just concentrate on who are the next pilgrims to join.

Even this is subject to dispute. n.o.body doubts that the cnidarians are a unitary group of pilgrims who all join up with each other 'before' they join anyone else. And n.o.body doubts the same of the ctenoph.o.r.es. The question is, in what order do they join each other and join us? All three logical possibilities have been supported. To make matters worse, there is a tiny phylum, the Placozoa, containing only a single genus, Trichoplax Trichoplax, and n.o.body knows for sure where to put Trichoplax Trichoplax. I shall follow the school of thought that says the cnidarians are the first to join us at Rendezvous 28 Rendezvous 28, then the ctenoph.o.r.es at Rendezvous 29 Rendezvous 29, then Trichoplax Trichoplax at at Rendezvous 30 Rendezvous 30. All this will become resolved definitively when more molecular data become available. This will be soon but, I fear, not soon enough for this book. Be warned that Rendezvous 28 Rendezvous 28 and and 29 29, as well as 30 and 31, could turn out to be in the wrong order.

Rendezvous 28 CNIDARIANS.

Our pilgrim band of worms and their descendants has now swelled to very large numbers, and we all pa.s.s on back to Rendezvous 28 Rendezvous 28 where we are joined by the cnidarians (the c is silent). They include the freshwater hydras and the more familiar marine sea anemones, corals and jellyfish, all very different from worms. Unlike the Bilateria, they are radially symmetrical about a central mouth. They have no obvious head, no front or rear, no left or right, only an up or down. where we are joined by the cnidarians (the c is silent). They include the freshwater hydras and the more familiar marine sea anemones, corals and jellyfish, all very different from worms. Unlike the Bilateria, they are radially symmetrical about a central mouth. They have no obvious head, no front or rear, no left or right, only an up or down.

What is the date of the rendezvous? Well, who knows? In order to draw rendezvous points in proportional positions in the diagrams that accompany them, it is necessary to set a date. But out here in deep time, there is so much uncertainty that we can do little but s.p.a.ce our dates out to the nearest 50 or even 100 million years. Anything smaller would convey a false sense of precision. Some authorities would disagree by hundreds of millions of years.

Because they are among our most distant animal cousins (some were once even confused with plants), the cnidarians are often regarded as very primitive. Of course this doesn't follow they have had the same time to evolve since Concestor 28 as we have. But it is true that they lack many of the features that we regard as advanced in an animal. They have no long-distance sense organs, their nervous system is a diffuse network, not urbanised into brain, ganglia or major nerve trunks, and their digestive organ is a single, usually uncomplicated cavity with only one opening, the mouth, which also does duty as a.n.u.s.

On the other hand, there aren't many animals who could claim to have redrawn the map of the world there aren't many animals who could claim to have redrawn the map of the world. Cnidarians make islands: islands you can live on; islands big enough to need, and accommodate, an airport. The Great Barrier Reef is more than 2,000 kilometres long. It was Charles Darwin himself who worked out how such coral reefs are formed, as we shall see in the Polypifer's Tale. Cnidarians also include the most dangerously venomous animals in the world, the extreme example being the box jellyfish, which oblige prudent Australian bathers to wear nylon bodystockings. The weapon cnidarians use is remarkable for various reasons, in addition to its formidable power. Unlike a snake's fangs, or the sting of a scorpion or a hornet, the jellyfish sting emerges from inside a cell as a miniature harpoon. Well, thousands of cells, called cnidocytes (or sometimes nematocysts, but this is strictly just one variety of cnidocyte), each with its own cell-sized harpoon called a cnida. Knide Knide is Greek for nettle, and it gives the Cnidaria their name. Not all of them are as dangerous to us as box jellyfish, and many are not even painful. When you touch the tentacles of a sea anemone, the 'sticky' feeling on your finger is the clutch of hundreds of tiny harpoons, each on the end of its own little thread, which attaches it to the anemone. is Greek for nettle, and it gives the Cnidaria their name. Not all of them are as dangerous to us as box jellyfish, and many are not even painful. When you touch the tentacles of a sea anemone, the 'sticky' feeling on your finger is the clutch of hundreds of tiny harpoons, each on the end of its own little thread, which attaches it to the anemone.

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Probably the most complicated piece of apparatus inside any cell.

Cross-section of a cnidarian harpoon.

The cnidarian harpoon is probably the most complicated piece of apparatus inside any cell anywhere in the animal or plant kingdoms. In the resting state, waiting to be launched, the harpoon is a coiled tube inside the cell, under pressure (osmotic pressure, if you want the details) waiting to be released. The hair trigger is indeed a tiny hair, the cnidocil, projecting outwards from the cell. When triggered, the cell bursts open, and the pressure turns the entire coiled mechanism inside out with great force, shooting into the body of the victim and injecting poison. Once triggered in this way, the harpoon cell is spent. It cannot be charged up again for re-use. But, as with most kinds of cell, new ones are being made all the time.

All cnidarians have cnidae, and only cnidarians have them. That is the next remarkable thing about them: they provide one of very few examples of an utterly unambiguous, single diagnostic characteristic of any major animal group. If you see an animal without any cnidae, it is not a cnidarian. If you see an animal with a cnida, it is a cnidarian. Actually, there is one exception, and it is as neat a case as you could want of an exception proving a rule. Sea slugs of the molluscan group called nudibranchs (they joined us along with almost everybody else at Rendezvous 26 Rendezvous 26) often have beautifully coloured tentacles on their backs, the kind of coloration that makes would-be predators back off. With good reason. In some species, these tentacles contain cnidocytes, identical to those of true cnidarians. But only Cnidaria are supposed to have cnidae, so what is going on? As I said, the exception proves the rule. The slug eats jellyfish, from which it pa.s.ses cnidocytes, intact and still working, to its own tentacles. Commandeered weapons, they are still capable of firing, in defence of the sea slug hence the bright warning coloration.

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Cnidarians join. The order of branching of the cnidarians (jellyfishes, corals, sea anemones and the like) and the ctenoph.o.r.es (comb jellies) is effectively unresolved. Most authors position either one or the other (or sometimes both) as the closest living relative of the bilaterally symmetrical animals. Certain molecular data hint that cnidarians may occupy this position. Unfortunately, the form and branching of sub-groups within the 9,000 or so cnidarian species is also disputed, but the fundamental division between lineages with or without an evolved medusa stage in their life cycle (see text) is widely accepted. The order of branching of the cnidarians (jellyfishes, corals, sea anemones and the like) and the ctenoph.o.r.es (comb jellies) is effectively unresolved. Most authors position either one or the other (or sometimes both) as the closest living relative of the bilaterally symmetrical animals. Certain molecular data hint that cnidarians may occupy this position. Unfortunately, the form and branching of sub-groups within the 9,000 or so cnidarian species is also disputed, but the fundamental division between lineages with or without an evolved medusa stage in their life cycle (see text) is widely accepted.

Images, left to right: white-spotted rose anemone ( white-spotted rose anemone (Urticina lofotensis); leptomedusan jellyfish (Aequorea sp.). sp.).

Cnidarians have two alternative body plans: the polyp and the medusa. A sea anemone or a Hydra Hydra is a typical polyp: sedentary, mouth uppermost, the opposite end fixed to the ground like a plant. They feed by waving tentacles about, harpooning small prey, then bringing the tentacle, complete with prey, to the mouth. A jellyfish is a typical medusa, swimming through the open sea by pulsing muscular contractions of the bell. The mouth of a jellyfish is in the centre, on the lower side. So you can think of a medusa as a polyp that has freed itself from the bottom and turned over to swim. Or you can think of a polyp as a medusa that has settled on its back with its tentacles uppermost. Many species of cnidarian have both polypoid and medusoid forms, alternating them through the life cycle, a bit like caterpillar and b.u.t.terfly. is a typical polyp: sedentary, mouth uppermost, the opposite end fixed to the ground like a plant. They feed by waving tentacles about, harpooning small prey, then bringing the tentacle, complete with prey, to the mouth. A jellyfish is a typical medusa, swimming through the open sea by pulsing muscular contractions of the bell. The mouth of a jellyfish is in the centre, on the lower side. So you can think of a medusa as a polyp that has freed itself from the bottom and turned over to swim. Or you can think of a polyp as a medusa that has settled on its back with its tentacles uppermost. Many species of cnidarian have both polypoid and medusoid forms, alternating them through the life cycle, a bit like caterpillar and b.u.t.terfly.

Polyps often reproduce by budding vegetatively, like plants. A new baby polyp grows on the side of a freshwater Hydra Hydra, eventually breaking off as a separate individual: a clone of the parent. Many marine relatives of Hydra Hydra do something similar, but the clone doesn't break off and a.s.sume a separate existence. It stays attached and becomes a branch, as in a plant. These 'colonial hydrozoans' branch and branch again, making it easy for us to understand why they were thought to be plants. Sometimes more than one kind of polyp grows on the same polyp tree, specialised for different roles, such as feeding, defence, or reproduction. You can think of them as a colony of polyps, but there is a sense in which they are all parts of one individual, for the tree is a clone: all the polyps have the same genes. Food caught by one polyp may be used by others, since their gastric cavities are all continuous. The branches of the tree and its main trunk are hollow tubes that you can think of as a shared stomach or maybe as a kind of circulatory system playing the role that in us is played by blood vessels. Some of the polyps bud off tiny medusae, which swim away like miniature jellyfish to reproduce s.e.xually and disperse the genes of the parent polyp tree to distant places. do something similar, but the clone doesn't break off and a.s.sume a separate existence. It stays attached and becomes a branch, as in a plant. These 'colonial hydrozoans' branch and branch again, making it easy for us to understand why they were thought to be plants. Sometimes more than one kind of polyp grows on the same polyp tree, specialised for different roles, such as feeding, defence, or reproduction. You can think of them as a colony of polyps, but there is a sense in which they are all parts of one individual, for the tree is a clone: all the polyps have the same genes. Food caught by one polyp may be used by others, since their gastric cavities are all continuous. The branches of the tree and its main trunk are hollow tubes that you can think of as a shared stomach or maybe as a kind of circulatory system playing the role that in us is played by blood vessels. Some of the polyps bud off tiny medusae, which swim away like miniature jellyfish to reproduce s.e.xually and disperse the genes of the parent polyp tree to distant places.

A group of cnidarians called the siphonoph.o.r.es have taken the colonial habit to an extreme. We can think of them as polyp trees which, instead of being fixed to a rock or a piece of seaweed, hang down either from one or a cl.u.s.ter of swimming medusae (which are, of course, members of the clone) or to a float at the surface. The Portuguese man-of-war Physalia Physalia has a large gas-filled float with a vertical sail on top. A complicated colony of polyps and tentacles dangles beneath. It doesn't swim but gets about through being blown by the wind. The smaller has a large gas-filled float with a vertical sail on top. A complicated colony of polyps and tentacles dangles beneath. It doesn't swim but gets about through being blown by the wind. The smaller Velella Velella is a flat, oval raft with a diagonally placed vertical sail. It too uses the wind to disperse, and its English names are Jack-sails-by-the-wind or by-the-wind-sailor. You often find the dried-up little rafts with their sails on the beach, where they usually lose their blue colour and seem to be made of whitish plastic. is a flat, oval raft with a diagonally placed vertical sail. It too uses the wind to disperse, and its English names are Jack-sails-by-the-wind or by-the-wind-sailor. You often find the dried-up little rafts with their sails on the beach, where they usually lose their blue colour and seem to be made of whitish plastic. Velella Velella resembles the true Portuguese man-of-war in that both sail by the wind. resembles the true Portuguese man-of-war in that both sail by the wind. Velella Velella and its relative and its relative Porpita Porpita are not siphonoph.o.r.e colonies, however, but single, highly modified polyps, hanging down from a float rather than sticking up from a rock are not siphonoph.o.r.e colonies, however, but single, highly modified polyps, hanging down from a float rather than sticking up from a rock (see plate 39) (see plate 39).

Many siphonoph.o.r.es can adjust their depth in the water, rather as bony fish do with their swim bladders, by secreting gas into the float, or releasing it. Some have a combination of floats and swimming medusae, and all have polyps and tentacles dangling beneath. The siphonoph.o.r.es are treated by E. O. Wilson, founder of the science of sociobiology, as one of the four pinnacles of social evolution (the others being the social insects, the social mammals and ourselves). This, then, is another superlative that one can attach to the Cnidaria. Except that, since the members of a colony are clones, genetically identical to each other, it is by no means clear that we should call them a colony rather than a single individual.

Hydrozoans see the medusa as a way for their genes to hop occasionally from one stable living place to another. Jellyfish could be said to take the medusoid form seriously, as what living is all about. Corals, by contrast, take sedentary living to the extreme lengths of building a hard, solid house that is destined to stay there for thousands of years. We shall take their tales in order.

THE JELLYFISH'S TALE Jellyfish ride the ocean currents as Jack-sails-by-the-wind. They don't pursue their prey, as a barracuda or squid might. Instead, they rely on their long, trailing, armed tentacles to trap planktonic creatures that are unlucky enough to b.u.mp into them. Jellyfish do swim, with the languorous heartbeat of the bell, but they are not swimming in any particular direction, at least as we would understand direction. Our understanding, however, is limited by our two dimensional trammels: we crawl over the surface of the land, and even when we take off into the third dimension it is only in order to crawl a bit faster in the other two. But in the sea, the third dimension is the most salient. It is the dimension in which travelling has the most effect. In addition to the steep pressure gradient with depth, there is a light gradient, complicated by a gradient of colour balance. But the light disappears anyway as day gives way to night. As we shall see, a planktonic animal's preferred depth changes dramatically with the 24-hour cycle.

During the Second World War, sonar operators looking for submarines were puzzled by what seemed to be a false bottom of the sea that rose towards the surface every evening, and sank back down again the next morning. It turned out to be the bulk of the plankton, millions of tiny crustaceans and other creatures, rising to feed near the surface at night, then sinking at morning. Why should they do this? The best guess seems to be that during the hours of daylight they are vulnerable to visually hunting predators such as fish and squids, so they seek the dark safety of the depths by day. Why, then, come to the surface at night, for it is a long journey that must consume a lot of energy? One student of the plankton has compared it to a human daily walking 25 miles each way, just to get breakfast.

The reason for visiting the surface is that food ultimately comes from the sun, via plants. The surface layers of the sea are unbroken green prairies, with microscopic single-celled algae in the role of waving gra.s.s. The surface is where the food ultimately is, and that is where the grazers, and those that feed on the grazers, and those that in turn feed on them, must be. But if it is safe to be there only by night because of visually hunting predators, a diurnal migration is exactly what the grazers and their small predators must undertake. And apparently they do. The 'prairie' itself doesn't migrate. If there were any sense in doing so, it should swim against the animal tide, for its whole raison d'etre raison d'etre is to catch sunlight at the surface during the day, and avoid being eaten. is to catch sunlight at the surface during the day, and avoid being eaten.

Whatever the reason, most of the animals in the plankton migrate down for the day and up for the night. The jellyfish, or many of them, follow the herds, like lions and hyenas tracking the wildebeest across the Mara and Serengeti plains. Although, unlike lions and hyenas, jellyfish don't target individual prey, even blindly trailing tentacles will benefit by following the herds, and this is one of the reasons jellyfish swim. Some species increase their catch rate by zigzagging about, again not individually targeting prey, but increasing the area swept by those tentacles with their batteries of lethal harpoons. Others just migrate up and down.

A different kind of migration has been described for the ma.s.sed jellyfish of 'Jellyfish Lake' on Mercherchar, one of the Palau Islands (an American colony in the western Pacific). The lake, which communicates underground with the sea and is therefore salty, is named after its huge population of jellyfish. There are several kinds, but the dominant one is Mastigias Mastigias, an estimated 20 million of them in a lake 2.5 kilometres long and 1.5 kilometres wide. All the jellyfish spend the night near the western end of the lake. When the sun rises in the east, they all swim straight towards it and therefore the eastern end of the lake. They stop before they reach the sh.o.r.e, for an interestingly simple reason. The trees fringing the sh.o.r.e cast a deep shadow, cutting off so much of the sun's light that the jellyfish's sun-seeking automatic pilot starts to drive them towards the now brighter west. As soon as they come out from the trees' shadow, however, they turn east again.

This internal conflict traps them around the line of the shadow, with the consequence (which I dare not think is more than coincidence) of keeping them a safe distance from the dangerously predatory sea anemones that line the sh.o.r.e itself. In the afternoon, the jellyfish follow the sun back to the western end of the lake, where the whole armada again becomes trapped at the shadow line of the trees (see plate 40) (see plate 40). When it becomes dark, they swim vertically up and down at the western end of the lake, until the dawn sun lures their automatic guidance system back towards the east. I don't know what they might gain from this remarkable twice-daily migration. The published explanation satisfies me too little to bear repet.i.tion. For now, the lesson of the tale must be that the living world offers much that we don't yet yet understand, and that is exciting in itself. understand, and that is exciting in itself.

THE POLYPIFER'S TALE All evolving creatures track changes in the world: changes in the weather, in temperature, rainfall and more complicated because they hit back in evolutionary time changes in other evolving lines such as predators and prey. Some evolving creatures alter, by their very presence, the world in which they live, and to which they must adapt. The oxygen we breathe was not there before green plants put it there. At first a poison, it provided radically changed conditions that most animal lineages were forced first to tolerate, and then to depend upon. On a shorter timescale, the trees in a mature forest inhabit a world that they themselves have created, over hundreds of years the time it takes to transform bare sand into climax forest. A climax forest is, of course, also a complex and rich environment to which huge numbers of other plant and animal species have become adapted.

Because the word 'coral' is used both for an organism and for the hard material that it builds, I shall indulge a fancy and adopt from Darwin the older word 'polypifer' for the coral organism that tells this tale. p.r.o.nounce it 'pol-lip-if-er', with the stress on lip. Coral organisms, or polypifers, transform their world, over a timespan of hundreds of thousands of years, by building on the dead skeletons of their own past generations to construct huge underwater mountains: wave-resisting ramparts. Before they die, corals combine with countless other corals to condition the world in which future corals will live. And not just future corals, but future generations of an enormous and intricate community of animals and plants. The idea of community will be the main message of this tale.

The picture reproduced in plate 41 shows Heron Island, the one island of the Great Barrier Reef that I have visited (twice). The houses dotted around the near end of the little island give an idea of scale. The huge pale area surrounding the island itself is the reef, of which the island is just the highest tip, covered with sand made of crushed coral (much of it having pa.s.sed through fish guts) in which vegetation of limited variety grows, supporting a similarly limited fauna of land animals. For objects that are entirely made by living creatures, coral reefs are big, and core drillings show some of them to be many hundreds of metres deep. Heron Island is just one of the more than 1,000 islands and nearly 3,000 reefs that const.i.tute the Great Barrier Reef, arcing round the north-east side of Australia for 2,000 kilometres. The Great Barrier Reef is often said with what veracity I don't know to be the only evidence of life on our planet that is large enough to be visible from outer s.p.a.ce. It is also said to be home to 30 per cent of the world's sea creatures, but again I am not sure quite what that means what is being counted? Never mind, the Great Barrier Reef is an utterly remarkable object, and it has been entirely built by the small sea anemone-like animals called corals or polypifers. The living polypifers occupy only the surface layers of a coral reef. Beneath them, to a depth of hundreds of metres in some oceanic atolls, are the skeletons of their predecessors, compacted to limestone.

Nowadays only corals build reefs, but in earlier geological eras they had no such monopoly. Reefs have at various times been built by algae, sponges, molluscs and tube worms too. The great success of coral organisms themselves seems to stem from their a.s.sociation with microscopic algae, which live inside their cells and photosynthesise in the sunlit shallows, to the eventual benefit of the corals. These algae, called zooxanth.e.l.lae, have a variety of different coloured pigments for trapping light, which accounts for the vividly photogenic appearance of coral reefs. It is not surprising that corals were once thought to be plants. They get much of their food in the same way as plants, and they compete for light as plants do. It is only to be expected that they would take on similar shapes. Moreover, their struggles to overshadow, and not be overshadowed, lead to the whole community of corals taking on something of the appearance of a forest canopy. And, like any forest, a coral reef is also home to a large community of other creatures.

Coral reefs hugely increase the 'ecos.p.a.ce' of an area. As my colleague Richard Southwood puts it in his book The Story of Life: The Story of Life: Where there would otherwise be a surface of rock or sand with a column of water above it, the reef provides a complex three-dimensional structure with a great amount of extra surface, with many cracks and small caves.

Forests do the same kind of thing, inflating the effective surface area available for biological activity and colonisation. Increased ecos.p.a.ce is the kind of thing we expect to find in complex ecological communities. Coral reefs are home to a huge variety of animals of all kinds, nestling in every corner and nook of the prodigious ecos.p.a.ce provided.

Something similar happens in the organs of a body. The human brain increases its effective area and hence its functional capacity by elaborate folding. It may be no accident that the 'brain coral' so strikingly resembles it.

Darwin himself was the first to understand how coral reefs are formed. His debut scientific book (after his travel book on the Voyage of the Beagle Voyage of the Beagle) was the treatise on Coral Reefs Coral Reefs that he published when only 33. Here is Darwin's problem as we would see it today, although he did not have access to most of the information that is relevant either to posing the problem or solving it. Darwin, indeed, was as astoundingly prescient in his theory of coral reefs as he was to be in his more famous theories of natural selection and s.e.xual selection. that he published when only 33. Here is Darwin's problem as we would see it today, although he did not have access to most of the information that is relevant either to posing the problem or solving it. Darwin, indeed, was as astoundingly prescient in his theory of coral reefs as he was to be in his more famous theories of natural selection and s.e.xual selection.

Corals can live only in shallow water. They depend upon the algae in their cells, and the algae of course need light. Shallow water is also good for the planktonic prey with which corals supplement their diet. Corals are denizens of sh.o.r.elines, and you can indeed find shallow 'fringing reefs' around tropical coasts. But what is puzzling about corals is that you can also find them surrounded by very deep water. Oceanic coral islands are the summits of lofty underwater mountains made by generations of dead corals. Barrier reefs are an intermediate category, following the line of a coast, but farther out than fringing reefs, and with deeper water between them and the sh.o.r.e. Even in the case of remote coral islands completely isolated in the deep ocean, the living corals are always in shallow water, close to the light where they and their algae can thrive. But the water is shallow only by courtesy of the generations of earlier corals on which they sit.

Darwin, as I say, didn't have all the information needed to realise the extent of the problem. It is only because people have drilled down into reefs, and found compacted coral to great depths, that we now know that coral atolls are the summits of towering underwater mountains made of ancient coral. In Darwin's time the prevailing theory was that atolls were superficial encrustations of coral on top of submerged volcanoes that lay only just below the surface. On this theory there was no problem to solve. Corals grew only in shallow water, and it was the volcanoes that gave them the perch they needed to find shallow water. But Darwin didn't believe it, even though he had no way of knowing the dead coral was so deep.

Darwin's second feat of prescience was his theory itself. He suggested that the sea bottom was continually subsiding in the vicinity of the atoll (while rising in other places, as he vividly knew from finding marine fossils high in the Andes). This was, of course, long before the theory of plate tectonics. Darwin was inspired by his mentor, the geologist Charles Lyell, who believed that parts of the Earth's crust rose and sank relative to one another. Darwin proposed that as the sea bottom subsided, it took the coral mountain down with it. Corals grew on top of the subsiding undersea mountain, just keeping pace with the subsidence in such a way that the summit was always near the surface of the sea, in the zone of light and prosperity. The mountain itself was just layer upon layer of dead corals which had once prospered in the sun. The oldest corals, at the base of the underwater mountain, probably began as a fringing reef of some forgotten piece of land or long-dead volcano. As the land gradually submerged beneath the water, the corals later became a barrier reef, at an increasing distance away from the receding coastline. With further subsidence the original land disappeared altogether, and the barrier reef became the basis for a prolonged extension of the underwater mountain for as long as the subsidence continued. Remote oceanic coral islands got their start perched on the top of volcanoes, the base of which slowly subsided in the same way. Darwin's idea is still substantially supported today, with the addition of plate tectonics to explain the subsidence.

A coral reef is a textbook example of a climax community, and this shall be the climax of the Polypifer's Tale. A community is a collection of species that have evolved to flourish in each other's presence. A rainforest is a community. So is a bog. So is a coral reef. Sometimes the same kind of community springs up in parallel in different parts of the world where the climate favours it. 'Mediterranean' communities have arisen not just around the Mediterranean Sea itself, but on the coasts of California, Chile, south-western Australia and the Cape region of South Africa. The particular species of plants found in these five regions are different, but the plant communities themselves are as characteristically 'Mediterranean' as, say, Tokyo and Los Angeles are recognisably 'urban sprawl'. And an equally characteristic fauna goes with the Mediterranean vegetation.

Tropical reef communities are like that. They vary in detail but are the same in essentials, whether we are talking about the South Pacific, the Indian Ocean, the Red Sea or the Caribbean. There are also temperate-zone reefs, which are somewhat different, but one very particular thing the two have in common is the remarkable phenomenon of cleaner fish a wonder which epitomises the sort of subtle intimacy that can arise in a climax ecological community.

A number of species of small fish, and some shrimps, ply a prosperous trade, harvesting nutritious parasites, or mucus, off the surfaces of larger fish, and in some cases even entering their mouths, picking their teeth and exiting through the gills. This argues for an astonis.h.i.+ng level of 'trust',1 but here my interest is more focused on cleaner fish as an example of a 'role' in a community. Individual cleaners typically have a so-called 'cleaning station', to which larger fish come to be serviced. The advantage of this to both parties is presumably a saving of time that might otherwise be spent searching for a cleaner or searching for a client. Site-tenacity also allows repeated meetings between individual cleaners and clients, which allows the all-important 'trust' to build up. These cleaning stations have been compared to barbers' shops but here my interest is more focused on cleaner fish as an example of a 'role' in a community. Individual cleaners typically have a so-called 'cleaning station', to which larger fish come to be serviced. The advantage of this to both parties is presumably a saving of time that might otherwise be spent searching for a cleaner or searching for a client. Site-tenacity also allows repeated meetings between individual cleaners and clients, which allows the all-important 'trust' to build up. These cleaning stations have been compared to barbers' shops (see plate 42) (see plate 42). It has been claimed though the evidence has more recently been disputed that if all the cleaners are removed from a reef, the general health of the fish on the reef nosedives.

In different parts of the world, the local cleaners have evolved independently, and are drawn from different groups of fish. On the Caribbean reefs, the cleaner trade is mostly filled by members of the goby family, which typically form small groups of cleaners. In the Pacific, on the other hand, the best-known cleaner is a wra.s.se, Labroides. L. dimidiatus Labroides. L. dimidiatus runs its 'barber's shop' by day, while runs its 'barber's shop' by day, while L. bicolor L. bicolor, so George Barlow, my colleague from Berkeley days tells me, services the nocturnal guild of fishes who take refuge in caves during the day. Such divvying-up of a trade among species is typical of a mature ecological community. Professor Barlow's book, The Cichlid Fishes The Cichlid Fishes, gives examples of freshwater species in the great lakes of Africa that have taken convergent steps towards the cleaner habit.

On tropical coral reefs, the almost fantastic levels of co-operation achieved between cleaner fish and 'client' is symbolic of the way an ecological community can sometimes simulate the intricate harmony of a single organism. Indeed, the resemblance is seductive too seductive. Herbivores depend on plants; carnivores depend on herbivores; without predation, population sizes would spiral out of control with disastrous results for all; without scavengers like burying beetles and bacteria, the world would sate with corpses, and manure would never be recycled into the plants. Without particular 'keystone' species, whose ident.i.ty is sometimes quite surprising, the whole community would 'collapse'. It is tempting to see each species as an organ in the super-organism that is the community.

To describe the forests of the world as its 'lungs' does no harm, and it might do some good if it encourages people to preserve them. But the rhetoric of holistic harmony can degenerate into a kind of dotty, Prince Charles-style mysticism. Indeed, the idea of a mystical 'balance of nature' often appeals to the same kind of airheads who go to quack doctors to 'balance their energy fields'. But there are profound differences between the way the organs of a body and the species of a community interact with each other in their respective domains to produce the appearance of a harmonious whole.

The parallel must be treated with great caution. Yet it is not completely without foundation. There is an ecology within the individual organism, a community of genes in the gene pool of a species. The forces that produce harmony among the parts of an organism's body are not wholly unlike the forces that produce the illusion of harmony in the species of a coral reef. There is balance in a rainforest, structure in a reef community, an elegant mes.h.i.+ng of parts that recalls co-adaptation within an animal body. In neither case is the balanced unit favoured as a unit as a unit by Darwinian selection. In both cases the balance comes about through selection at a lower level. Selection doesn't favour a harmonious whole. Instead, harmonious parts flourish in the presence of each other, and the illusion of a harmonious whole emerges. by Darwinian selection. In both cases the balance comes about through selection at a lower level. Selection doesn't favour a harmonious whole. Instead, harmonious parts flourish in the presence of each other, and the illusion of a harmonious whole emerges.

Carnivores flourish in the presence of herbivores, and herbivores flourish in the presence of plants. But what about the other way around? Do plants flourish in the presence of herbivores? Do herbivores flourish in the presence of carnivores? Do animals and plants need enemies to eat them in order to flourish? Not in the straightforward way suggested by the rhetoric of some ecological activists. No creature normally benefits from being eaten. But gra.s.ses that can withstand being cropped better than rival plants really do flourish in the presence of grazers on the principle of 'my enemy's enemy'. And something like the same story might be told of victims of parasites and predators, although here the story is more complicated. It is still misleading to say that a community 'needs' its parasites and predators like a polar bear needs its liver or its teeth. But the 'enemy's enemy' principle does lead to something like the same result. It can be right to see a community of species, such as a coral reef, as a kind of balanced ent.i.ty that is potentially threatened by removal of its parts.

This idea of community, as made up of lower-level units that flourish in the presence of each other, pervades life. Even within the single cell, the principle applies. Most animal cells house communities of bacteria so comprehensively integrated into the smooth working of the cell that their bacterial origins have only recently become understood. Mitochondria, once free-living bacteria, are as essential to the workings of our cells as our cells are to them. Their genes have flourished in the presence of ours, as ours have flourished in the presence of theirs. Plant cells by themselves are incapable of photosynthesis. That chemical wizardry is performed by guest workers, originally bacteria and now re-labelled chloroplasts. Plant eaters, such as ruminants and termites, are themselves largely incapable of digesting cellulose. But they are good at finding and chewing plants (see the Mixotrich's Tale). The gap in the market offered by their plant-filled guts is exploited by symbiotic micro-organisms that possess the biochemical expertise necessary to digest plant material efficiently. Creatures with complementary skills flourish in each other's presence.

What I want to add to that familiar point is that the process is mirrored at the level of every species' 'own' genes. The entire genome of a polar bear or a penguin, of a caiman or a guanaco, is an ecological community of genes that flourish in each other's presence. The immediate arena of this flouris.h.i.+ng is the interior of an individual's cells. But the long-term arena is the gene pool of the species. Given s.e.xual reproduction, the gene pool is the habitat of every gene as it is recopied and recombined down the generations.

1 The evolutionary problems of evolving 'trust' are interesting, but I have already dealt with the matter in The evolutionary problems of evolving 'trust' are interesting, but I have already dealt with the matter in The Selfish Gene The Selfish Gene, so must refrain from repeating myself here.

Rendezvous 29 CTENOPh.o.r.eS.

The ctenoph.o.r.es, who join us at Rendezvous 29 Rendezvous 29, are some of the most beautiful of all the animal pilgrims. A superficial resemblance has led them to be wrongly cla.s.sified as jellyfish. They used to be placed in the same phylum, which was known as the Coelenterata, celebrating their shared characteristic, the fact that the main body cavity is also the digestive chamber. They also have a simple nerve net, like the Cnidaria, and their bodies are likewise built from (disputably) only two layers of tissue. The balance of modern evidence suggests, however, that the cnidarians are closer cousins to us than they are to the ctenoph.o.r.es: another way of saying that the cnidarians join the pilgrimage 'before' the ctenoph.o.r.es do. I don't feel confident enough of this, however, to quote a date for the event.

Ctenoph.o.r.e in Greek means 'comb-bearer'. The 'combs' are prominent rows of hair-like cilia, whose beating propels these delicate creatures in place of the pulsating muscles that do the same for the superficially similar jellyfish. It is not a fast system of propulsion, but it presumably serves adequately, not for actively chasing prey but for the same kind of undirected improvement in capture rate that the jellyfish achieve. Because of their resemblance to jellyfish, and their delicate jelly-like consistency, the ctenoph.o.r.es are known in English as comb jellies. There aren't many species of them only about 100 but the total number of individuals is not small, and they beautify, by any standards, all the oceans of the world. Waves of synchronised motion pa.s.s up the comb rows in eerie iridescence.

Ctenoph.o.r.es are predatory but, like jellyfish, they rely on prey pa.s.sively b.u.mping into their tentacles. Although their tentacles look like those of jellyfish, they have no cnidocytes. Instead, they have their own make of 'la.s.so cells', which discharge a kind of glue instead of sharp, poisonous harpoons. Perhaps we could see a ctenoph.o.r.e as a kind of alternative way of being a jellyfish. Some of them, however, are far from bell-shaped. The ravis.h.i.+ngly beautiful Cestum veneris Cestum veneris is one of those rare animals whose English and Latin names mean exactly the same thing, Venus's girdle, and no wonder: the body is a long, s.h.i.+mmering, ethereally beautiful ribbon, too good for a G.o.ddess is one of those rare animals whose English and Latin names mean exactly the same thing, Venus's girdle, and no wonder: the body is a long, s.h.i.+mmering, ethereally beautiful ribbon, too good for a G.o.ddess (see plate 43) (see plate 43). Notice that, although the Venus's girdle is long and thin like a worm, the 'worm' has no head or tail end, but is mirrored about the middle, where the mouth is the 'buckle' of the girdle. It is still radially (or strictly biradially) symmetrical.

[image]

Ctenoph.o.r.es join. The bilaterally symmetrical animals, together with the cnidarians and ctenoph.o.r.es, are sometimes referred to as the 'Eumetazoa'. Following some molecular studies, the 100 known species of ctenoph.o.r.e are here placed as the most distant relatives of the rest, but this position is not definitive. The bilaterally symmetrical animals, together with the cnidarians and ctenoph.o.r.es, are sometimes referred to as the 'Eumetazoa'. Following some molecular studies, the 100 known species of ctenoph.o.r.e are here placed as the most distant relatives of the rest, but this position is not definitive.

Image: Beroe Beroe sp. sp.

Rendezvous 30 PLACOZOANS.

Here is an enigmatic little animal: Trichoplax adhaerens Trichoplax adhaerens, the only known species in its entire phylum, the Placozoa which, of course, doesn't necessarily mean it is the only one. I should mention that in 1896 a second placozo