They lack tissues and organs and have no symmetry. They are flagellated cells that line the pores that beat to provide a current of water to bring in and trap food particles.
Amoeboid cells secrete the skeletal elements of the sponge.
More complex than sponges they have tissues. The polyps are usually attached to the substrate and may be solitary or colonial. Common form of Hydrozoans are Hydras which live in shallow, freshwater ponds, less than 1 cm tall and have no medusa stage. Corals are usually colonial and much smaller, they secrete a hard skeleton of calcium carbonate with many small cups surrounding the polyps.