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 for ever. This is the simplest form of animal currently known: a sponge.