are generally classified into four classes : [ 5 ] Hydrozoa Scyphozoa Cubozoa Anthozoa Number of species [ 4 ] 3,600 228 42 6,100 Examples Hydra , siphonophores Jellyfish Box jellies Sea anemones , corals , sea pens Cells found in mesoglea No Yes Yes Yes Nematocysts in exodermis No Yes