Use paper lunch bags to model the simplest of animals:
Sponge: a plain bag
It has an inside and an outside and it just sits there.
Cnidarian: a bag with the top cut into fringes
Flatworm: a flat bag, stapled/taped shut, with a straw stuck into it in one spot
One opening, through which both food enters and waste leaves.
Nematod/Roundworm: a flat bag, stapled/taped shut, with a straw passing through in two spots
Separate spots for food to enter and waste to leave.
