Panarthropoda

The segmented body plan is useful.  You can speed up quite a bit by having long muscles under the skin which pull both against your insides (hydrostatically) but also against your jointed outsides.  You get a little faster, and avoid getting eaten.

For some time now, you’ve had cells which are aware of light changes.  They help you know if you are hiding under a rock under out in the open, for example.  As you get bigger and faster, some of these cells happen to develop in pits in your skin.  This is very good because they begin to give you awareness of the direction light is coming from instead of just the general light level.  This makes you more aware of your environment and thus more effective, and more likely to reproduce, and so the genes for ‘light cells in pits’ spread.  Other possible arrangements are found by random changes, and the genes causing the better ones are more likely to spread through the population. Eventually, the light cells are found in deep, circular pits, giving good quality information about light in a very specific direction.  After perhaps as few as several hundred more generations, there is a membrane of fine skin over the light pits, and the pits contain clear fluid.  The simple eye has arrived.

At the same time, your segments develop more shape which gives you more speed.  Like the ridges on the bottom of a pair of walking boots, those among you with rough surfaces at the bottom of the segments have more grip, and are more successful.  Evolution ruthlessly leaves behind the least efficient, until only those with extrusions that look like clawed feet on the sides of the bases of the segments are left.

You are tough, fast, and able to see what is in front of you.  In fact, you can now eat small worms, and you certainly do.  You are a hunting worm, which preys upon smaller worms - as well as being hunted by larger worms.

You might prefer to concentrate on eating other animals, and stay lightweight and fast, and never mind that some of you are eaten.  Alternatively, you might prefer to stiffen up that skin into armour plates, and accept that this will make you slower because it will also make you less vulnerable?

Armour me

Speed please

Onychophora

You become a larger and faster worm.  By modern times, you can move at about 0.1 miles per hour, and have become an efficient hunter of insects and other small creatures.

You are still clearly a worm, because although you have what look like legs on the sides of your segments, they are only ever moved in pairs, so there is no real ‘walking’ gait.  You actually move much like an ancestral worm, but those paired ‘legs’ have gained the ability to flex slightly and actively grip on slippery surfaces.

You are a velvet worm.

That's as far as you can evolve here!  
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Unsegmented Cuticle Worms

You are still a worm with a stiff skin.  The worms which developed segmented body plans are running away and leaving you as the easy prey to be eaten.

You need some kind of protection.  How about a retractable head?  That way the stingers will not damage the most vulnerable part of you, and you may still escape?

Seriously?  A retractable head?  Oh go on.
Don’t be silly.

Ecdysozoa

One lucky little prehistoric worm with a gene error expresses a hard protein in its skin, called chitin.  In many other situations this would a problem, making the animal stiff, and slow, and easy prey, and so the gene would die out, but at this point in time it’s just the thing.

Animals with this gene are slightly protected against assault by hunting jellyfish, and after a few generations, selection pressure causes a group to develop with a cuticle - a harder covering than normal ectoderm cells, which aids escape from stinging, grabbing predators.

As you grow, however, this cuticle can’t grow with you, so another mutation rapidly spreads through the population - at intervals, the cuticle is disconnected from the cells producing it, and a new, softer, larger cuticle develops underneath it.  Eventually the old cuticle is sloughed off, and you stretch out inside your new armour.  You are briefly vulnerable while the new cuticle softens, but behaviours such as hiding or burrowing rapidly develop.

At this time, a ‘worm’ is basically a long slim tube comprised of two types of muscle.  Circular ones, which squeeze from side to side and make you lengthen, and a single longitudinal muscle which can shorten the whole length of you.  The first set of muscles contract in groups to extend the front, then the back, then the front again.  It’s simple but it works.  Slowly.  Differentiating the groups more strongly might improve the separation of contractions, and enable a slightly faster gait.

Would you like to develop rings which break up your body into segments?  They will cost a little to make…

I’m smooth
I’m chunky

Protostomia

You develop a complete, or partial but long, gut, which forms backwards from your mouth.

You have a front and a back, and you are symmetric from side to side.  You look pretty worm like, and live in the bottom of the ocean, where you develop the ability to creep along and consume whatever is in front of you.  This makes you quite recognisable as a primitive animal.  You are  several millimetres long when fully grown.

Unfortunately, that's still quite small as far as some other creatures go.  The various jellyfish which developed before you would quite like to grab you, sting you to death, and eat you.  You're going to need to be tough.  Or fast.  Which is it?

Toughen up.
Shift up a gear.

Bilateria

Selection pressure makes you get bigger and bigger, because being big makes you less likely to be eaten.  You develop another kind of cell which is neither a specialised feeding cell for the inside surface or a specialised outside cell for the outside surface, but a specialised inside cell to fill you up.

Do you remember how we described the big ball of cells as a dented ball?    Despite the new kind of cell, the dents just get deeper and deeper.  Eventually, they get so deep that the dent becomes more than a mouth, it actually punctures through the back of the animal, and the animal becomes a tube.  

This tube shape will be very efficient, as you can keep pushing food in at one end and let what you can’t use fall out the back.  Obviously, you only want to expend the energy to develop sensory, food-pulling cells at one end, as it would be inefficient to have them at both, so the next choice is simple: pick a direction.  Is the first dent going to be the mouth or the anus?

Mouth?

Anus?

Hydrozoa

Many of you tiny cnidarians band together, to form what appears to be a single organism.  You are mostly, even in a group, quite tiny, but a few of you form colonies which are up to a couple of metres across.

Interestingly, many of your forms are not just a group acting together, but actually acting as a single organism, with the internal cavities joining into those of other individuals, in a conjoined form.

A few of you have given up on the 'normal' jellyfish form and spend only a tiny part of your life like that, having expanded the proportion of the larval stage to almost all of the time.  Medusa are single individuals which join to others and grow to form colonies.  As a result, only the single organisms can swim effectively and the colonies are either anchored to the sea bed, like fire corals, or floating freely with the wind to drive them, like Portugese Man o' War.

Some of you have adapted to form symbiotic relationships with crabs, and eat the crabs scraps in return for providing a stinging defence.  A few of you have even adapted to live in fresh water.

That's as far as you can evolve here!  
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Scyphozoa

You have become one of the 'true' jellyfish (although there is no particular reason to consider them more specifically jellyfishy than any one of the sister clades you would have reached with a slightly different choice).

You swim freely in the world's oceans, although you're not really strong enough to swim against any noticeable current.  You are fairly a passive consumer of plankton and very small creatures, such as fish and crustacean, which stray into your stinging, poisonous tentacles.

You have a two part lifecycle - the familiar free floating 'medusa' stage, and a larval stage which sits quietly on the sea bed, and either transforms into a medusa or buds one or more medusa off it.

You are found from the shallow warm waters of the world's shore-bound seas, to the dark, cold depths of the oceans, in hundred of species, limited only by your need to stay in salt water... for now.  

Who knows what changes the next mutation will bring?


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Shell-less and flap-less jellies

Being pretty small, and slow, you become a popular food for other organisms.  You can swim away, but not quickly enough to escape.  On the other hand, being small, it is pretty inexpensive to make lots and lots of offspring, and some of them will survive.

Even without a shell to make you safe or a a flap to make you fast, sheer cheapness and numbers can carry you through.

You could, however, improve your chances a bit by creating a colony: lots of similar creatures all working together as one big organism, instead of individual creatures.  Of course, teamwork isn't always a success.

What's it going to be?

Lone Ranger?
Dirty Dozen?

Cubozoa

Whoa!  That made a huge difference.  The Box Jellyfish are the fastest jellyfish by far, able to swim a metre in as little as ten seconds when going long distance, or sprinting short distances at a rate of a metre in half a second - around human walking pace.  When you are hunting small prawns and fish, that's plenty fast enough as they tend to hide rather than flee, and then your long tentacles can reach in and sting them.

Your tentacles increase in length and venom, and you become a pretty amazing hunting machine.  Some of you grow up to two metres across and three metres long!  

You develop two kinds of eyes, some of which are 'full' eyes, giving you 360 degree vision all around you (and in the ocean, that means above and below, too), as well as many more simple eyes that show only dark and light, which are perfect for spotting nearby prey movements.

You appear to have developed a full vision system, shown by your ability to swim towards and away from objects of interest or danger, and swim around obstacles before you can touch them.  This indicates some kind of awareness, even intelligence.  That's amazing!

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Shell-less jellies

Without a shell, perhaps being able to move faster would help?  In order to do that, you need to expel water with more force.  Simple physics suggests that increasing the size or strength of part of the body doing the squeezing is what you need.  Jellyfish bodies are not really adaptable for strength, so a bigger squeezer is the best bet.

If you mutated to have an extra flap of body around the bottom of your umbrella section, that might work?  Of course, bigger body means less is available for tentacles.

Extra flap
No flap

Conulariida

One little mutation, and your soft body starts producing a hard shell.  Now you move much more slowly, but you are much harder to eat.

You start living the other way up to the other jellyfish, with a single anchoring structure at one end of your four-sided cone, and tentacles at the other.  It is thought that you stay still for periods of time, then move, living slowly, growing quite big in the rich waters of the Cambrian era.

Unfortunately, as the temperature cools between the Cambrian and Triassic eras, heavy shells cost too much to create and transport, and you are out-competed in this niche by other jellyfish who didn't grow a shell.

You became extinct in the Lower Triassic.  Sorry.

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Jellyfish

Hmm.  It's all very well being free-floating stinging blobs, drifting into food-rich waters, but that can't be quite enough.  What happens if you drift into a still water area where there is no food?

Over time, those of you who wiggle a bit when you get stuck have a better survival rate, and pass on your genes, and the wiggle improves until you are able to swim, albeit very slowly.

This takes a very very long time, and during that time, other animals have developed.  You are now also a marvellous passing snack for other creatures.

Perhaps you need a defence of some kind?  Would a shell work?

Shelled jellies
Shell-less jellies

Pennatulacea

Remaining entirely soft, you float in the currents which bring you your food, but retain the ability to let go of the sea bed and float away in the pursuit of better positioning.

You can grow quite large, up to two metres, and live quite long, over a hundred years.

Often brightly coloured, you resemble a feather, or quill, and have been named 'sea pens'.

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Alcyonacea

Your insides produce small needles of minerals, called spicules or sclerites, and these enable you do deter predators enough to become long living and successful.

You are still soft enough to wave in the ocean currents though, and you are known as 'soft corals'. You come in many colours and are found all over the world.

Soft Alcyonaria

Without a skeleton, you are still a tasty snack.  You could try a 'skeleton-lite' approach, and grow little needles of rock instead?  These are called sclerites.

Sclerites?
No sclerites?

Coenothecalia

A mutation causes you to express a form of calcium carbonate called Aragonite, and this makes your insides crunchy and tough, enabling you to grow larger, and preventing predators from eating you.

You spread across the world, and are a common form of coral on coral reefs.

You have a beautiful blue colour underneath your living flesh, and are called Blue Coral.

Alcyonaria

You have repeating groups of eight sections, and you live on the ocean floor.  A stabby cnidarian, living off passing phytoplankton, in the high competition of the seas.

In order to obtain as much food as possible, when you mature, you anchor yourself in the best food providing currents you can find.  However, this can leave you exposed to predators.  You are, after all, still just a squishy bundle of grub to something a lot bigger than you, even if you have little stingers.

Perhaps something that makes a hard skeleton will make it more difficult to chew you up?

Get a skeleton?
Do without.

Scleractinia

A calcium carbonate exoskeleton provides an excellent defence (at least until parrotfish evolve), but limits the size of the individual polyp.  Many tiny polyps living close together tend to make lots of calcium carbonate which forms into the massive chunks we call reefs.

These can grow and grow and grow, very slowly but inexorably, so that the longest reef today is the Great Barrier Reef, which is two and a half thousand kilometres long.  The biggest reef area is the South China Sea Reef Bank, nearly nine thousand square kilometers (half the size of Wales).

Other species of coral create smaller, distinctly structured shapes, such as Brain Coral or Fan Coral.  Some have colonised the deep ocean, hundreds of meters down, where they may have developed other energy gathering approaches such as methane collection.

In fact, the only thing in two billion years to trouble the coral family is the rising water temperature, which makes calcium carbonate more soluable and may eventually dissolve their entire ecosystem.

Corallimorpharia

As time passes, and the 'big mouth' strategy starts to pay off, you grow a larger and larger, flattened disk shaped 'mouth' on top of a small column, which anchors you to the sea bed.

You become mushroom shaped, except that the top of the mushroom is covered in tiny stinging tentacles arranged in radial rows out from the central mouth.  You are more mouth than anything else.

You are quite difficult to eat, because any animal getting too close is stung, and so you live a good length of time.  Some of your kind evolve even further, to express poisons which make you a poor choice of food indeed.

You are a corallimorph.

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Soft Symbiotic Polyps

Well, if you're choosing to stay soft, you're going to have to live faster or quieter.  You need to reproduce before you're spotted and eaten.

Maximising your eating area, by growing a wider and wider mouth would enable you to capture more prey, and be able to reproduce earlier.  Alternatively, staying small, with the ability to pull your tentacles in and look just like a shiny rock might be effective.

Anything in between these two approaches won't work.  A medium sized mouth that can't be hidden away will leave you obvious.  Time to choose again.


Big mouth?
Small mouth?

Symbiotic Polyps

You are eaten by a zoantharia.  It thinks you are tasty food.  Luckily, over time, it has been letting lifeforms like you live on inside it for a little while, because you can continue photosynthesising and making sugar while inside.

In fact, it has evolved to provide the chemicals you need as food to extend your life inside it.

This adaptation has now reached the point where it can contain multiple photosynthesising algae, and they can live on inside it indefinitely.

You have entered a symbiotic relationship, and will never live outside again, getting food and protection, for the price of a few sugars.  You are part of a symbiotic polyp, and will evolve along with it from now on.

As you are soft and small, and now sweet, you are quite vulnerable to anything bigger and hungry that happens to be near you.  An adaptation to create some kind of armour might be useful right about now, although it comes at a cost.

Skeleton?
No Skeleton?

Symbiotic Polyps

You eat some photosynthesising algae.  It’s tasty food.  However, over time, you have found that allowing the algae to live for a little while inside you makes them even better, as they can continue photosynthesising and making sugars inside you.

In fact, you have already evolved to provide the chemicals the photosynthesising algae needs as food to extends its life inside you, and maximise your meal

This adaptation has now reached the point where you can contain multiple photosynthesising algae and they can live on inside you indefinitely.

You have entered a symbiotic relationship, and will come to rely on your guest, providing food and protection, for the price of a few sugars.

As you are soft and small, and now sweet, you are quite vulnerable to anything bigger and hungry that happens to be near you.  An adaptation to create some kind of armour might be useful right about now, although it comes at a cost.

Skeleton?
No Skeleton?

Antipatharia

You live in the sunless seas of the deep ocean, and a few shallow but sunless sounds.

You grow very slowly, but you live a very long time indeed.  Some of you are known to be over four thousand years old.

Your outer shell grows thick, and hard, and dark.  Inside, your cnidarian colours are very bright indeed.

Your shell is sought after by humans as it can be polished to a brilliant shine, and is hard to get.

You are a Stony Coral.

Actiniaria

A small mouth, sitting atop a column which attaches you to the rock, staying in areas where the currents bring the food right past you, and spearing whatever comes too close, you are a pretty effective hunter.

Although you are good to eat for some species, such as sea stars, sea slugs, and turtles, most species won't eat you because you sting them.  You hide in the cracks in the rocks, and withdraw your tentacles to look inconspicuous, when the need arises.

If you really need to get away, you can actually let go of the rock, and, by flexing your body around your hydrostatic ball of stomach, swim to another location.  Some species in fact have adapted to spend their entire lives floating.

You are a sea anemone.

Zoantharia

You have repeating groups of six sections, and you live on the ocean floor.  A stabby cnidarian, living off passing phytoplankton, in the high competition of the seas.

The competition to survive is always fierce, and every bit of energy you can get is worth having.  Some of the food floating around you is able to make its own food from sunlight - and you can't do that.  Maybe you'd like to team up with them and get a boost?

Obviously, this option is only useful if you are currently living in the shallow, sunlit seas.  The speciation occurs because not everyone can take advantage of the opportunity.

Sunny side up.
It's dark down here.

Archaeplastidia

You are eaten by a corticata.  It thinks you are tasty food.  Luckily, over time, it has been letting lifeforms like you live on inside it for a little while, because you can continue photosynthesising and making sugar while inside.

In fact, it has evolved to provide the chemicals you need as food to extend your life inside it.

This adaptation has now reached the point where it can contain multiple photosynthesing bacteria, and they can live on inside it indefinitely.

You have entered a symbiotic relationship, and will never live outside again, getting food and protection, for the price of a few sugars.

Inside your host, you photosynthesise using a chemical called 'chlorophyll a', which turns you a blue-green colour and gives you a supply of sugars all the time you are in sunlight.  It doesn't absorb all the sunlight though - only a part - the rest is wasted.

The usual copy-error effect can give you the ability to make a slightly different version of chlorophyll which will work using a different part of the sunlight spectrum, but it means you'll make less of chlorophyll a.  Interested?

I'll go with two.
The one I have is just fine.

Anthozoa

You stick to the sea floor, spearing passing food and slowly growing.  This is a quite successful strategy, and enables you to grown (slowly) into one of the biggest entities around at this time.  As you grow, however, you will need a strategy for organising your body so that you remain efficient.  No point in growing a new 'mouth' immediately behind another one.

A set of genes arrives, by chance, that cause you to grow in certain patterns, instead of simply amorphously.  Among other things, they impose radial symmetry on your polyps.  Too many degrees of symmetry will be over complex.  Too few will be inefficient. Having some genes for one kind of symmetry mixed with some for another will not be as useful as picking one type and sticking with it, and eventually all the examples of your kind with a mixture will be out competed.  The two successful strategies turn out to be eight-fold symmetry and six-fold symmetry.

Higher?

Lower?

• 
  

Placozoans

You had muscles, and nerves, and specialised cells within you.  But they're not enabling you to survive, floating in the water.  In an example of evolution reversing direction, you lose all those features in favour of a flattened, simplified body, which crawls very slowly across smooth surfaces using flagella.

You are 'under the radar' for a food source for the other animals.  You survive, almost unnoticed into the present day, and are only recognised as an animal in the late 1800s.

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Solid, smooth, stingless animals

This is terrible.  The comb jellies and the jellyfish, both close relatives, are now both hunting you down and eating you!  You need to do something to avoid the threat.

In a highly stressed situation, for a small group, evolution occurs quickly, because there are fewer individuals.  Thus the likelihood of a gene spreading across a whole population is therefore higher.

There are typically two evolutionary choices: you can chuck out things that aren't needed to increase your efficiency, or you can evolve rapidly in a dwindling population and find something that works.

The risk of the first one is that you might become so simple that you lose the advantages you have got to date.  The risk of the second is that you don't find a solution before the population becomes so small that it isn't viable.

Increase complexity?

Decrease complexity?

Cnidaria

Another random mutation, and a marvellous new cell type appears.  Initially, these will have been only slightly useful, but over millennia they have mutated and mutated, with each change which makes them more powerful spreading throughout the population until they become amazing.

The cnida cell sits on the surface of the outer skin and contains a fine tube, coiled up, waiting.  When the outer surface of the cell is touched, a part of the cell containing calcium ions ruptures and releases calcium into the interior of the cell.  This causes a massive inrush of water via osmosis, and the change in pressure forces the tube out straight, becoming several times longer than the original cell.  Spear fishing!

Any small food particle, such as plankton, which is carried close enough past you gets impaled and is then available to slowly pull in and digest.

In order to improve your efficiency, you could now do one of two things.  Stay still, somewhere food is plentiful (hopefully staying that way) or swim with the current, where you can find food sources.

Stay still.
Swim free.

Solid, smooth animals

Hey!  Hang on a minute!  Your sister species, the comb jellies, with their efficient filter fringes, have grown quite big and are now eating you!  You'll need to develop something pretty special to defend yourself and maybe help you to catch things to eat yourself.

If you express a chemical that other animals will find painful to the touch, they'll leave you alone.

Fear my stingers!
Try something else.