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?

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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.

That's as far as you can evolve here!  
You can go back to the start with the link above, or share what you became with the icons below.

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.