Lisa Herr, age 14, of Conestoga, Pennsylvania, for her question:
What is a planarian?
He looks like a small flat worm, which is one reason why the planarian is classed as a flatworm. We rarely notice him for he spends the day hiding on the shady side of rocks and water weeds. But in the early 1960s, the planarian became suddenly famous. Researchers discovered that the shy little flatworm is a remarkable student. Of all things.
The flattish, wormy planarian is about half an inch long. If he could be enlarged to human size, he would be mistaken for a Halloween ghost, sheathed in a black sheet. He has a wide triangular head and his little tail tapers to a point. He has two round eye¬spots that looks like bright eye holes in his black Halloween sheet.
Sometimes he lives in a patch of very wet soil. But he prefers a coolish stream, where the water is fresh and not too muddy. He will even settle for a bubbling spring. During the night, he glides through his watery world, searching for meat. During the day he dozes in the shade.
Though he is very shy, it is possible to coax a planarian out of hiding. The bait is a bit of meat, dangled from a string. Choose a spot on the shady side of a submerged rock and if he is there, he will come forth to dine. And where there is one planarian, there are bound to be more. In some mysterious way, news of the banquet travels around the pond or along the stream. Soon another and another planarian arrive to sample your bait.
The little eye spots do not see clearly, but they can perceive brightness and shadow. The wormy little body is covered with slimy mucus. This leaves a slippery trail, which he uses to glide over stones, rocks and waterweeds. He is covered with tiny hairs called cilia, which he waves back and forth to carry him along his slithery carpet of mucus.
The little black spook happens to be a favorite study among biology students. He has a fabulous talent for regeneration. This means that he can regenerate or regrow a lost part of his body. He can survive being cut into three or four pieces. He even can survive a beheading.
When a planarian loses his head, both parts are regenerated. The lost head grows a new body, the old body grows a new head. Among most animals, beheading is fatal. But for the planarian, the drastic operation results in two planarians. Most animals could not survive having the head sliced in half down the middle. The planarian regenerates each half. The result is a planarian with two heads attached to one tapering body.
Researchers also have discovered that the planarian is no dummy. He can learn to respond to light signals. With careful training he can be taught to go through a maze to find food. And, when an educated planarian is cut in half, the regenerated planarians learn faster than the original. Though small, the planarian certainly is a most remarkable creature.