Pat Stone, age l4, of Indianapolis, Ind., for his question:
What is a hydra?
The original hydra was a snake haired monster of mythology. When one of its nine heads was removed it grew two more. The living hydra of biology is too small to be seen without a lens, but it can outdo this legend. then cut into l,000 pieces it can regrow into l,000 hydras.
You can keep a collection of hydras in a sample of pond or lake water. You will need a good magnifying glass or a hand lens to see them in detail, and their wondrous antics are well worth watching. The average hydra looks like a skinny tree trunk with five or seven scrawny branches. It is, however, an animal, and the tree trunk is a body no bigger than a quarter or half inch snip of sewing thread. The branches are tentacles.
The hydra is a coelentrate, which means a hollow stomached animal. When hungry, of course, everyorie seems to have a hollow stomach, but the body of a hydra is almost entirely hollow stomach. His trunk is a digestion sack with a small opening at the top. Isis tentacles are for gathering food and stuffing it into the hollow stomach. The food enters and the wastes from digestion leave by the same door.
The fine tentacles have microscopic stinging hairs to subdue copepods, water fleas and other food items. The hydra may be brown or black, gray or green, depending upon what he eats. On a diet of algae he may become vivid green, and when reri copepods are on his regular menu he becomes bright orange red. When famished he is colorless.
As a rule, he stands on one spot, using his tentacles to trawl for food. But the hydra is a born acrobat. He can drift through the water upside down or travel by somersaults. It is fascinating to watch him bend his tentacles to stand upside down and loop over again and again. He also can stretch himself out long and thin or shrink to a small bump.
The hydra may reproduce by egg cells or by budding. The offspring may start as a bud on the parent's side and when grown may separate itself or remain attached. Sometimes 30 to 40 generations of hydras remain joined together in a branching thread a foot long. And every hydra has a miraculous talent for regrowing any lost part of its small body.
This regeneration is no problem to the hydra's simplified body. It has no head or limbs, no brain or sense organs and its trunk is just a double layer of cells, an outer skin and an inner layer of digestive tissue. If cut and recut into l,000 slices, each fragm.ent can regrow itself into an adult, healthy hydra.