Cnidaria

 There are four major groups of cnidarians: Anthozoa, which includes true corals, anemones, and sea pens; Cubozoa, the amazing box jellies with complex eyes and potent toxins; Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae; and Scyphozoa, the true jellyfish.
 

The name Cnidaria comes from the Greek word "cnidos", which means stinging nettle. Casually touching many cnidarians will make it clear how they got their name when their nematocysts eject barbed threads tipped with poison.

 Polyps, in contrast, are usually sessile (they don't move). They have tubular bodies; one end is attached to the substrate (such as a rock), and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals.  In colonies, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense.

HYDROZAS

Perhaps the best-known hydrozoan, is Hydra, pictured above. Hydra never goes through a medusoid stage, and spends its entire life as a polyp.
The "Portuguese man-o'war" and "by-the-wind-sailors" that often wash up on beaches are examples of these unusual colonial hydrozoans.   As with the other cnidaria, hydra capture prey with nematocysts.  Nematocysts are coiled stingers with are used to paralyze prey, they are located in the cnidocytes, as pictured below.  Just a touch will trigger a nematocyst.  Once fired the hydra will wrap its tenticles around its prey and draw it into it's gastrovascular cavity.  This cavity contains enzymes that will break down the prey so the the hydra can absorb it into its mouth.

The sting of the "Portuguesse man-o'war"  is very painful to man and can cause serious effects, including fever, shock, and interference with heart and lung action.  Beneath the water are clusters of polyps, from which hang tentacles of up to 165 feet (about 50 meters) in length.  If stung, remove tentacles and rinse with water, emergency medical attention may be needed.

Hydras can reproduce both sexually and asexually.  Asexual reproduction occurs during warm weather when small buds grow on the tentacles and then break off and grow independently.  Sexual reproduction occurs  in the fall.  Eggs are produced in ovaries and sperm are produced in the testes.  Some times this can occur in the same individual, often it occurs in separate individuals.  Either way, sperm is released into the water and eventually find its way to the ovary of a nearby individual.
 

SCYPHOZOA

Scyphozoans include most of the jellyfish familiar to beach-goers.  True jellyfish are graceful, and sometimes deadly creatures. Their stings may cause skin rashes, muscle cramps.  Jellyfish range in size from a mere twelve millimeters to more than two meters across. The largest is Cyanea arctica, which may have tentacles over 40m long! Despite their often enormous size, jellyfish have no head, no skeleton, and no special organs for respiration or excretion. Their life cycle involves an alternation between sessile polyp phase and a free-swimming medusa stage, though the medusa stage, shown in the picture here usually predominates.  The adult medusa releases sperm and egg into the water, where fertilization occurs.  The resulting zygote develops into a blastula.  The blastula then develops into a ciliated larva called a planula, which settles and attaches to the ocean bottom.  The unattached end develops a mouth, tentacles, becoming a polyp.  As the polyp grows, it forms stacks of medusae, each of which buds off and develops into a jellyfish.

If stung by a jelly fish, it is often helpful to rinse with vinegar or sprinkle with meat tenderizer!

http://www.mbari.org/~kraskoff/medusae2.htm
http://www.aloha.com/~lifeguards/portugue.html
http://www.aqua.org/animals/species/jellies.html
http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html
http://tolweb.org/tree?group=Cnidaria&contgroup=Animals