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?