sea anemone belongs to the phylum cnidaria, and cnidaria as well as ctenophora are the only phylum division with diploblastics cells.Therefore the answer is sea anemone(they do not have coelom
sea anemone is a coelenterate.so it is diploblastic and acoelomate.Organisms from annelida onwards are coelomate.Aschelminthes are pseudocoelomate.rest are acoelomate.From platyhelminthes onwards the organisms are triploblastic.