Judy Irwin, age 10, of Victoria, B.C., for her question:
Does a jellyfish start life as a plant?
There are big and little jelly fish; some live in salt water and some live in fresh water. All of them have jelly like umbrellas with long streamers trailing below. They move by jet propulsion. The umbrella opens, taking in a bagful of water. Then it contracts, squirting out the water in a jet. The force of the jet pushes the jellyfish ahead. Most of the creatures are but a few inches wide but in the far northern Atlantic; there lives a jellyfish whose umbrella is more than seven feet wide. His trailing tentacles may be 120 feet long and he may tip the scales at one ton.
No one would mistake an adult jellyfish for a plant. But if you watched his life cycle, you might make this mistake during his kindergarten days. There are special names for each of the amazing stages through which he passes. As an adult, he is called a medusa, pt the proper time, the medusa releases a number of fertilized eggs from under its umbrella. Each little egg is called a planula. It is covered with short, fine hairs which it uses to swim through the water.
The planula, who travels under its own power, could not be mistaken for a plant. It settles to the bottom and stands up on a little trunk and bulging top. It is now called a polyp and if we did not know that it was one stage in the life history of an animal, we might easily mistake it for a plant. Some jellyfish polyps even produce little buds. However, they are animals and the buds become adult jellyfish.
Certain jellyfish go through still more plant like stages. The polyp becomes a tall trunk ringed with garter ridges. It is now called a strobilia. The ridges soon develop into fringes and the strobilia looks for all the world like a graceful underwater plant which, of course it is not,
The fringes which circle the trunk become deeper and the atrobilia might remind you of a pile of daisies or perhaps a stack of frilly . doilies. Actually, it is a pile of unborn jellyfish,
When it is ready, the flat little doily on top of the pile breaks away. The center of the doily becomes the 3ellified blob which is tile center of the animal's body. The fringed edges become the tentacles, At this stage, the baby jellyfish is called an ephyra. It moves its little umbrella and swims off into the water.
In a short while, the second doily pulls off from the pile. One little ephyra follows another and soon they are all swimming, floating and drifting through the water. Most of them will be eaten by hungry sea dwellers. A few lucky ones will live to grow up and become medusas, or adult jellyfish.