It was the dominant phylum in the late Precambrian period.  
It was the dominant phylum in the late Precambrian period.  Creatures in it phylum have bodies consisting of an internal gastrovascular cavity, a soft, pliable middle body layer known as the mesoglea, and an external dermis.  Its members often live in two phases, a sessile, asexual polyp and a free-moving, sexual medusa.  FTP, name this phylum that includes hydra and jellyfish.
It was the dominant phylum in the late Precambrian period.  
Cnidaria or Coelenterata