Forest Preserve District of Cook County, Illinois
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Jellyfish and Their Kin
Nature Bulletin No. 278-A    October 14, 1967
Forest Preserve District of Cook County
Richard B. Ogilvie, President
Roland F. Eisenbeis, Supt. of Conservation

The creatures that live in the sea are entirely different from those in fresh water. An inlander, a "landlubber", is fascinated by them. It is a thrilling experience to find one's first starfish, or a flower-like sea anemone. Among the strangest of marine animals are the jellyfish, which are not fish at all but relatives of the sea anemones and of the many kinds of coral that form rock-like skeletons and slowly build such enormous structures as coral reefs and coral atolls.

There are many, many kinds of jellyfish. Some are tiny; others are as large as half a grapefruit; a few rare ones are as large as a bushel basket and have been known to be seven feet in diameter. Some are transparent; others are brown, pink, blue or white; and some are phosphorescent. The common kinds are shaped like a bell or like an umbrella, with a fringe around the edge, and some of them have numerous long streamers that trail behind. The mouth and stomach are where the handle of an umbrella would be. The animal slowly swims by contracting the bell or half closing the umbrella, thus forcing it forward, and then leisurely expanding it. Ocean bathers avoid the big ones because their tentacles, used to paralyze smaller marine animals, cause a painful sting.

Jellyfish are almost exclusively marine but they do have a few freshwater relatives. Every few years, someone is astonished to find a pond or a small stream swarming with thousands of small whitish jellyfish about a half-inch diameter. Then they disappear and their like may not be seen again until years later and perhaps hundreds of miles away. In Illinois they have been found two or three times in the last fifty years. This haphazard occurrence is because these free-swimming jellyfish, like many of their relatives, do not come from other jellyfish but from small plant-like creatures, called hydroids, which live permanently attached to underwater objects. These hydroids, in turn, develop from the eggs of jellyfish and this process is called "alternation of generations".

The commonest and best known freshwater relatives are the little Hydras: simple sac-like animals that live attached to submerged plants, sticks and stones, or cling to the underside of the surface film of water, or sometimes float free. When expanded, each looks like a bit of thread, a half-inch or an inch long, with a tiny sticky foot on one end and the other end frayed out. When disturbed, it contracts into a speck of jelly the size of a pinhead. The frayed end, dangling lazily in the water, is really a circlet of 6 or 8 slender arms, or tentacles, around the mouth opening. They are used in capturing smaller food animals and have special stinging cells which explode at the slightest touch, paralyzing the victim with poisoned darts. The other arms then aid in holding the prey and cramming it through the mouth into the capacious central cavity. There the water flea, tiny worm, young insect or baby fish is digested and the useless parts cast out through the mouth. A hydra has no eyes, ears or brain -- only a vague net of nerves which cause the animal to expand or contract. It travels by gliding slowly along a surface, or by using its tentacles to loop along like a "measuring worm"', or by turning "slow motion handsprings".

Eight kinds of hydras are widely distributed over the United States. One is bright green because of an alga that lives in its body, two are brownish, and the other five range from pinkish-gray to orange-red. Each has two methods of reproduction: by buds and by eggs, usually at different seasons. A bud starts as a small' pouch on the side of the sac, develops a mouth, and finally breaks loose from the parent. The eggs, produced one at a time, hatch and grow into new hydras.

"jellyfish, like some people, has no backbone."

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