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?