Phylum Cnidaria

Moon jellyfish, one of the numerous species of cnidarian.

How a nematocyst fires

This phylum has quite a number of species included, but the most well-known of them is the group known as jellyfish. However, though all true jellyfish are cnidarians, not all cnidarians are jellyfish. though there are several characteristics unique to this phylum, one of them is post important to the general observer: the cnidae, of which the nematocysts are the most common. Nematocysts are stinging cells and they're what make brushing up against the wrong side of a jelly hurt so much. The nematocysts are contained within specialized cells called cnidocytes, which can be triggered in several ways, including direct contact or even just a change in water salinity. When the nematocysts are fired out, their barbs and spines will stick inside anything they come in contact with. One common action of the nematocycts is paralyzing the prey to make it easier to digest. Since cnidarians have soft, jelly-like bodies it's very easy for their body to be damaged, so it would be devastating for the animal to ingest struggling prey.

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)

This group has two general body plans: the medusa and the polyp. The medusa is the body form most associated with the jellyfish. The poly is what is seen in such creatures as coral and the hyrda. Many species have both, alternating between to body forms and also alternating between types of reproduction. While medusae are basically exclusively associated with sexual reproduction, polyps are capable of a number of things. Polyps in some species reproduce sexually, while others reproduce asexually through either budding or something called strobilation, and still other species may do all three. Strobilation, for the uninitiated, creates a number of genetically identical individuals (which will become medusae) at the mouth of the polyp is a sort of stacked-cup arrangement. In some species, the reproductive cycle goes something like this: medusa > sexual reproduction > ciliated larva (planula) > attached polyp > asexual budding > asexual strobilation > juvenile medusa (ephyra) > medusa. Reproduction is one of the major ways that cnidarians are classified.

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.