In all animals except sponges, coelenterates (sea anemones/jelly fish) and cnidarians (comb jellies) that development proceeds further. A third layer of cells, mesoderm, forms between the ectoderm and endoderm, and forms many complex body organs (heart, kidneys, etc.), as well as muscle tissue. These animals are now mobile as adults. Their gut also breaks through to the other end, so that they have both a mouth and an anus. These animals are called “bilaterians”, and they are the phyla that radiate in the Cambrian.