Sponge
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesoglea) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
The Thing About Jellyfish
Suzy postulates that Franny's death was the result of Irukandji syndrome after Suzy learns about jellyfish on a field trip to an unnamed aquarium, which Benjamin states is the New England Aquarium in the end notes. While researching the subject, Suzy identifies the following real-life jellyfish experts she thinks could ask for help: