Phylum Coelenterata or Cnidaria: Cnidarians are the first true metazoans that have attained tissue grade of construction. Cnidaria is a phylum of diploblastic, acoelomatic animals that possess stinging cells and a blind sac gastrovascular cavity. The phylum evolved in the Proterozoic era, more than 600 million years ago. The term Coelenterata was coined by Leuckart