|Moon jellyfish, one of the numerous species of cnidarian.|
|How a nematocyst fires|
Cnidarians are a simple sort of animal, not even having a complete digestive tract. Their mouth also serves the same purpose as the anus found in other animals. This makes digestion someone inefficient since opening its mouth to ingest new food can cause the animal to loose a fair amount of partially digested food and any potential energy it could obtain from it. The do have a simple nerve net, muscle tissue, sensory organs, and numerous other things that the previously mentioned phyla do not have.
|Medusa (left) and polyp (right)|
There are a number of Classes within this phylum: Scyphozoa, Cubozoa, Hydrozoa, Myxozoa, and Anthozoa. Class Scyphozoa includes those jellyfish that virtually everyone knows about. They alternate between medusa and polyp, and one of their most distinguishing characteristics is their use of strobilation. Some in this group have zooxanthellae, or algae that live within the animal's tissue. Class Cubozoa is the box jellyfish, which are rather infamous for their painful sting. Their medusae are box-like and they also, believe it or not, have complex eyes with lenses. They have polyps and medusae, but do not strobilate. Class hydrozoa includes such species as the hyrda and unique colonials like the Portugese man-of-war. These species have a life cycle that is dominated by the polyp rather than the medusa. Class Myxozoa consists of strange parasites that I won't talk about for simplicity's sake. Class Anthozoa includes all of the corals and the anemones, polyp-only species that may be colonial (coral) or singular (anemones).
I could go on forever about the differences between they types and such, but I'm going to cut it short here.
Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under a Creative Commons license or are copyright free: one, two, three.