The fossil remains of the earliest known ancestors that show a striking resemblance to the modern day jelly fish, have been found in the rocks of a Utah mountainside that once lay at the bottom of what was once a prehistoric tropical sea.
Dating back to over 505 million years, these delicate fossils show us that the evolution to our modern day jelly indicates that the jelly fish has been so ideally suited to its environment that even over the course of millions of years, mother nature has seen little need to change the physiology of this highly successful organism.
They are made up with an outer layer (epidermis) which covers the outer body and an inner layer (gastrodermis) which serves as the lining for the gut. A thick jellylike substance called mesoglea lies between the epidermis and the gastrodermis.
There are a wide variety of jellyfish and some can even reach a size of 7 feet with tentacles reaching lengths greater than 100 feet!
Jellyfish populate every major ocean of the world and are capable of withstanding a wide range of temperatures and salinity. Most can be found in shallow coastal waters, but a have been discovered at depths of 12,000 feet.
The jellyfish is easily recognized by its simple bell shaped form, and many tentacles that are used to paralyze their microscopic prey. Its fragile physiology is made up of 94 - 98% of water. However, it's simple form belies a complicated reproduction cycle that goes through many stages before it reaches the familiar shape that we all know.
Adult jellyfish reproduce sexually (being both male and female individuals) and the male will release his sperm into the water which the female receives the swimming sperm through her mouth which is situated underneath the bell shaped part, where they will be fertilized. The eggs will start to develop either inside the female or they may be held in brood pouches which can be found on the "oral arms" of the jellyfish.
Eventually the eggs will hatch into the short lived free swimming planula larva.
The larva will drift in the water and slowly settle to the bottom and becomes attached to a hard surface and then begins its next stage of development a scyphistomanta or polyp. This is what is known as "the sessile stage", meaning that the polyp is fixed and not free swimming.
This polyp will then go onto to develop its own polyp colony known as a strobilating scyphistomanta and will be connected together by tubes for feeding. This is also known as a sessile stage because the polyps are still not free swimming at this time.
When the sea starts to become warmer during the spring, the polyps will then give off thousands of tiny, jelly fish by budding asexually. This stage is known as ephra. The tiny jelly fish will continue to get bigger until they become adults and become the jelly fish (Medusa) that we all recognize.
San Francisco Chronicle Nov 5 2007: http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2007/11/05/MN6NT5D99.DTL