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

****:JELLYFISH AND THEIR KIN

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."