Excerpts from Jim Conrad's
from the May 26, 2013 Newsletter issued from the Frio Canyon Nature Education Center in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
PLANARIAN IN A JAR
During the cold months when not much was flowering or fruiting I explored life in the little Dry Frio River's shallow waters behind the cabin, focusing especially on algae and protozoa. I placed jars of Dry Frio water on my window sills and through the months it's been fascinating watching how these little biosystems evolve, some species dying out, others proliferating, and each jar seems to have developed differently. One jar's water is murky green because of an alga population explosion, but water in the jar next is clear with a modest little sprig of some kind of flowering plant, and a third jar is in-between, but home to thousands of darting-about microbes who show up as specks when the morning sun shines through it.
This week in one jar, and only one, the walls became peppered with tiny freshwater snails and minute, brownish, worms that moved across the jar's walls. I fished one of the worms out, coaxed it onto a microscope slide, and below you can see it :
That picture is not taken through a microscope because he was too large -- about ¼ inch (7mm) when extended. The photograph shows him drawn up to maybe a quarter of that length. Still, the field marks telling us what this animal is are easy to see: He's a flat worm whose head bears two conspicuous eyespots, or ocelli, giving the impression that they're "crossed," and the head is "eared" like an arrow with a rounded point.
Most everyone who has taken a biology class can instantly recognize that this is a planarian, for biology classes the world over study planaria because they're so easy to keep alive and possess an interesting nervous system. A planarian split lengthwise or crosswise will regenerate into two separate individuals. In fact, very small cut-out pieces of certain planaria as little as 1/279th of the organism can regenerate back into a complete organism over the course of a few weeks. You may have seen pictures of a planarian with its head cut in half down its center, resulting in a planarian with two complete heads.
Many kinds of planaria exist, some terrestrial, many marine, some freshwater like ours. They move by beating cilia on their undersurfaces, enabling them to glide along on a film of mucus. Some species can move by undulating their bodies. I read that freshwater planaria eat protozoa. Ours appeared to be grazing the jar's inner walls along with the snails. However, planaria don't have the snail's raspy tongues with which to scrape algae off the glass. Instead, planaria have a single opening or mouth at the end of a muscular tube, the pharynx, which is extended when feeding. It's hard to see how such a snaky pharynx might be used on a class wall. Maybe ours were just out exploring among the snails. Other individual planaria were down in the detritus at the jar's bottom, which was rich in one-celled protozoa, and a flexible pharynx might easily come in handy there.
During my hermit days in Mississippi I ran into an invasive planarian species about half a foot long. You might enjoy reading about that one, seeing it, and comparing it to our little jar-wall traveler at http://www.backyardnature.net/n/a/planaria.htm.