It stared at me. I stared at it.
The planarian is a small (1/2” or less) and simple flatworm. It lives in rivers, freshwater ponds and saltwater, gliding across the bottom as it looks for prey (small worms, crustaceans) . The two eyespots are sensitive to light, which the planarian prefers to avoid. On the sides of its triangular head, the “ears” are actually sensitive to touch and to chemicals.
On its belly is the mouth. The planarian extend its long tubular pharynx out through the mouth and secretes digestive enzymes. These begin to chemically break up the prey. The pharynx then sucks up the bits. These go into a simple gut. The intestinal cells lining the gut complete the digestion. Nutrients are dispersed throughout the body by a highly branched system of tubes. Excretory pores along the sides release the chemical waste products.
Planaria breathe through their skin, receiving oxygen and releasing carbon dioxide. They move by the beating of small hair-like projections from the cells on their bellies, or via muscle contractions. The muscles are a part of the body membrane. The nervous system is very simple: two large bodies of nerve cells (ganglions) behind the “ears”, and nerve cords and smaller ganglions in a ladder-like configuration down the body.
They are hermaphrodites, having both testes and ovaries, but they must mate to reproduce, exchanging each other’s sperm. The eggs develop internally, are shed in capsules, and several weeks later hatch into mini-adults.
So why was I excited to see one? The last time I saw one was in a biology class, in a petri dish, where it was being cut in half. Planaria have the amazing ability to regenerate missing parts. You could cut one up into over 100 pieces and each one would grow into an intact, fully functional planarian. How does this happen? About one fifth of the cells in its body are stem cells, capable of making any of the various tissues and organs, much as the cells of an early embryo (embryonic stem cells) can do.
Today, researchers studying planaria have identified a protein, TOR, which is essential for regeneration. Without it, no growth can occur. Researchers are looking for ways to inactivate TOR in cancer cells, to stop them from multiplying and to activate TOR where needed to stop degeneration caused by disease or Alzheimer’s.
Eve Broughton was educated at UC. Berkeley and lives in Whitethorn.