StoryImage( ‘/Images/Story//Auto-img-11594608399710.jpg’, ‘Image courtesy of www.issgallery.org’, ‘Hydra, the mythological nine-headed dragon, which was slain by Hercules, was a montrous beast, and the small, primitive animal found in freshwater resembles its namesake in some ways.’);
How would you feel being called nothing but an animated stomach, with perhaps a load of superfluous jelly to make you appear to be fat and prosperous? If this scenario sounds attractive, it is suggested you read up on a primitive group of animals called the coelenterates, a word that means hollow intestine in Latin.
Coelenterates, or cnidarians, as they are sometimes called because of their stinging cells, represent the most primitive group of multicellular animals and have been around in the waters of planet earth in excess of 500 million years. Included in this group are such familiar marine forms as jellyfish, corals, sea anemones, sea pens, and small polyp-like forms that resemble plants more than animals. A few live on today in freshwater, namely species of Hydra (both the scientific and common name of the animal) and a few small jellyfish.
For years, zoologists recognized sponges as the most primitive animals, but they are now placed in a separate group known as the Parazoa, meaning alongside of animals. Sponges are multicellular, but the cells are not organized into tissues as they are in the rest of the animal kingdom.
Hydra, the mythological nine-headed dragon, which was slain by Hercules, was a monstrous beast, and the small, primitive animal found in freshwater resembles its namesake in some ways. Hydras main claim to fame is that it is studied by all freshmen zoology students as a basic representative of its group of animals.
If freshwater vegetation is collected from a non-polluted pond and placed in a glass container of water, hydras may be observed clinging to vegetation or the sides of the container. Though small, they may be readily observed with the unaided eye if you look closely. They usually resemble a short piece of thread, frayed at one end to form a number of long tentacles. Both the body and the tentacles are capable of contraction and expansion because of primitive muscle fibers running throughout the body.
Hydras main occupation in life is the capturing of food to stuff into its digestive cavity. On all parts of its body, but especially numerous on the tentacles, are batteries of complex stinging cells called nematocysts. Each stinging cell has a protruding spine that acts as a triggering mechanism.
When a small animal is unfortunate enough to brush against the body of a hydra or one of its tentacles the stinging cells are fired, and the victim is harpooned. Venom is injected, and the prey is quickly paralyzed. It is then transferred to the gaping mouth by one or more of the tentacles. The edges of the mouth close around the organism, and it is unceremoniously stuffed into the digestive cavity. Indigestible particles are regurgitated out through the mouth with a sudden squirt-like action. All coelentereates have only one opening to the digestive tract, and that might be a reason they are so disagreeable and mean. I have watched gluttonous hydras cram themselves so full of mosquito wrigglers or water fleas that their bodies resembled small spherical beads topped off by the tentacles.
When a hydra grows too large, it solves the problem by asexually producing a bud from its body wall that will eventually break off and develop into a completely new hydra. In the fall, hydra resorts to the more complicated process of sexual reproduction. One animal may produce both male and female sex cells, or the sexes may be separate. In either event, an egg cell is released into the water where it is fertilized by a sperm cell from the same or another individual. A tough shell is secreted around the fertilized egg, forming a cyst-like structure that drops to the bottom of the pond, where it safely passes the winter.
Early zoologists were amazed when they discovered Hydra could regenerate itself when cut into many small pieces, and that they could graft one hydra onto another in the same manner horticulturists graft trees.
This remarkable power of regeneration and regrowth is a great advantage for this delicate animal in the struggle for existence. It is easy to understand how they have been able to exist and prosper for hundreds of millions of years.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.
From the Sept. 27-Oct.3, 2006, issue