Which of these animals doesn't have a heart?

There are many species without hearts. All species belonging to the phyla Porifera, Echinoderms, Cnidarians, Ctenophores, and Platyhelminthes are insensitive, as they have the lowest level of organization. Most major animal groups (phyla) lack hearts. Sponges lack hearts; they have no organs at all. Cnidarians, ctenophores (jellyfish), and Platyhelminthes (flatworms) lack hearts and generally use their gastrovascular cavities (guts) to circulate nutrients throughout their bodies.

Some of them don't even have brains; they may have ganglia (groups of nerve cells or cell bodies). In sponges, cells don't even coordinate with other cells, which is why when a cell or part of a sponge falls off, it becomes a new sponge because the cells are independent and can live and reproduce.