Comb Jellies

Increasingly frilly edges, covered with sticky cells develop on your outer surfaces and enable you to gain a rapid advantage in water.  You experience a substantial increase in efficiency and are able to grow rapidly bigger than other animals similar to you.

Eventually, the frilly edges, known as cilia, become big enough that you can actually use them to swim with, instead of just floating with the current.  This enables you to become a predator, following and eating other animals, which are actually your close relatives!  This is a common feature of predation: the best animals to eat are ones which are fairly closely related, because they share the same chemical make-up and so are the best match you to your needs.

Eventually, the large variation of size means that you become many different species, each tuned to a particular form of hunt.  Some of the largest of you effectively form swimming mouths, with stiffened cilia acting as teeth.

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 animals

As the ball of cells gets bigger and bigger, it loses structural integrity, like a partially inflated ball.  Smaller balls are OK, but bigger ones are likely to dent.  That’s what happens to you.

Luckily, this turns out to have an advantage!  At the back of the dent, water is trapped, and food particles don’t just whoosh past, they stop, and can be eaten.  This is the beginning of some sort of digestive tract.  The dent becomes deeper and deeper, and begins to look more like a sac, edged with cells which specialise in pulling in food and lined with cells that specialise in eating food.  Anything that can’t be digested has to come out the same way.

Further improvements develop, giving you the ability to pull the ‘mouth’ closed.  It’s a rudimentary set of muscles and nerves - not a central nervous system yet, but something that might one day develop into one. 

The same types of specialised cells which line the mouth could occur in other parts of the body, creating a sort of fringed edge which would help - as the water goes around your edges, you can maximise your surface area with a frill, which will increase the proportion of useful food items you can grab out of the stream.

Get frilly?
Stay smooth?

Sponges

Improvements in your ability to take in nutrients are developed by mutations which cause first dips, then channels, then pores, in your surface.  Bit by bit, you become more efficient at filtering the water flowing past you - indeed, through you.

Your reproduction process is the release of eggs and sperm cells, which meet in the water and form larvae, which swim until they settle to a new site and grow there.  This is the simplest form of animal currently known, although there are hints that tube worms may be even older and simpler.

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.

Animalia

This is the beginning of the animals, although you would be hard put to realise it.  At present you are an amorphous blob of cells, with digestive capabilities spread over the external surfaces.  You survive only in sea water, where sufficient nutrients pass by you that you can live.  You are, oddly enough, rather like a giant, multicellular bacterium.  It's odd how certain approaches to life appear again and again, at different scales and with different mechanisms, but the same behaviours.

And like all forms of life, if you don't find a niche that no-one else can use and occupy it, you will be squeezed out by other forms which do.  You need to improve your ability to take in nutrients.  

The strategy so far has been bigger and bigger balls of cells, but that is reaching its limits.  Perhaps the balls could split into loosely connected clumps and allow fluids to flow between them?  Or just keep getting bigger and bigger?

Let the water flow through me.
Keep me strong and solid