The larval form of a member of phylum Cnidaria (or Coelenterata), which includes jellyfish, anemones, coral, and hydras. The planula larva is microscopic and sausage-shaped with cilia, and does not eat. The animal is a planula larva between the time it develops from a fertilized egg until it settles onto a substrate and becomes sessile. The larval stage is the only time in the life cycle of a Cnidarian when the animal is motile and can travel great distances.