What makes hydras different from other cnidarians and how do the cells of a hydra get oxygen?

Hydras spend their life as a Polyp, a sessile organism,
with most found in freshwater, rather than saltwater. They are radially symmetrical and
feed on small organisms in the water by stinging them with their Cnidoblasts and
ingesting the paralyzed prey through their mouth. This is accomplished with a simple
reflex action carried out by their tentacles which push the food into the mouth. Because
their body plan is relatively simple, with two cell layers and a jelly-like layer in
between, their cells are in constant contact with their watery environment. Oxygen,
which is dissolved in the water, can be absorbed into their cells by the process of
diffusion. The oxygen will spread through the body slowly by this process. However,
their tiny size and metabolic needs are supported by this method of oxygen
transport.