Hydra (phylum Cnidaria) is diploblastic — its body wall is composed of only two germ layers: the outer ectoderm and the inner endoderm, separated by a non-cellular mesoglea. Crabs (Arthropoda), squid (Mollusca), and earthworms (Annelida) are all triploblastic — they develop three germ layers (ectoderm, mesoderm, endoderm). Diploblasty is characteristic of phyla Cnidaria and Porifera.