Planaria are small, free-living flatworms best known for regenerating lost body parts. They’re also among the most rewarding animals for amateur microscopists. They’re big enough to find by eye, cheap to keep, and have a surprising amount of visible anatomy: eyespots, a branched gut, and a feeding tube that slides out from the middle of the body.
What planaria are
“Planaria” is the everyday name for freshwater flatworms in the order Tricladida. The word comes from the old genus name Planaria, and many classroom species belong to genera such as Dugesia, Girardia and Schmidtea. They’re flat, soft-bodied and unsegmented. Most have a spade- or arrow-shaped head with two dark eyespots that make them look cross-eyed.
They are free-living, not parasitic. They live under stones, leaves and wood in ponds, streams and springs, eat small invertebrates and dead animal matter, and avoid light.
Can you see planaria with your eyes?
Yes. Most common freshwater planaria are a few millimetres long, and larger individuals can reach a centimetre or more, depending on species and how well fed they are. You’ll see them as small brown, black, grey or white slivers gliding over a surface. Their internal features need magnification.
Finding or buying planaria
Bait trapping is the classic method:
- Find a clean stream, spring or pond with stones and leaf litter.
- Put a small piece of raw liver, egg yolk or meat in a jar or a container with holes, and sink it near the bottom in shallow water.
- Check it after an hour or two, or leave it for longer. Planaria are often drawn to the bait and can be picked off with a soft brush or pipette.
You can also turn over stones and leaves and look on the undersides. Get permission where needed, and don’t collect from protected sites.
Biological suppliers sell live planaria cultures for schools. This is the easiest option if you want a known species and a predictable start.
Keeping planaria for observation
- Use clean, unchlorinated water: spring water, aged dechlorinated tap water, or water from their habitat.
- Keep them in a shallow dish or jar in a cool, dark place. They dislike strong light and warm water.
- Feed them small pieces of liver, boiled egg yolk or similar about once a week. Remove leftover food after a few hours, then change the water, because it fouls quickly.
- Avoid soap residue and metal containers.
Which microscope to use
A stereo (dissecting) microscope at about 10x to 40x is the best tool for live planaria. You see the whole animal moving in a dish of water, with light from above or below.
A compound microscope at 40x to 100x can show more internal detail if you gently confine a planarian under a coverslip with very little water. It won’t survive long that way. Keep the session short, and give it a well slide or plenty of water if you want to return it to the culture.
A hand lens at around 10x is enough to see the eyespots and the way they glide.
What to look for
The head, eyespots and auricles
The head usually has two eyespots (ocelli). These are cups of pigment with light-sensitive cells, so they sense light direction rather than forming images. Many species also have ear-like flaps on the sides of the head called auricles. These carry chemical and touch receptors, and you may see the planarian waving its head side to side to “smell” its surroundings.
Gliding movement
Planaria glide smoothly rather than wriggle. They move on a layer of mucus, driven by cilia on the underside of the body, with muscle contractions taking over when they move faster. Under low power, look for a faint trail and watch how the head steers.
The pharynx and gut
The mouth is on the underside, around the middle of the body, not at the head end. A muscular, tube-like pharynx can extend out of it to feed. At rest, it shows as a paler, elongated area in the centre of the body.
The gut has three main branches, one forward and two back, which is where the name “triclad” comes from. Each branch has many side pockets. In a well-fed animal under transmitted light, the gut often shows up as a darker branching pattern.
Make the gut visible with food colouring
A simple trick: mix a little food colouring into a small amount of egg yolk or liver paste and feed it to the planaria. After they’ve eaten, the coloured food fills the gut branches and makes the pattern much easier to see. Change their water afterwards.
Behaviour you can test gently
- Light avoidance: shade half a dish and they tend to gather on the dark side.
- Water currents: a gentle puff from a pipette near the head or tail can produce different responses.
- Chemical cues: they move towards food odours.
Prepared slides: whole mounts and cross-sections
Biological suppliers sell prepared slides that show detail impossible to see in a live animal:
- Whole mounts (WM), often with the gut injected with dye, show the full branched digestive system.
- Cross-sections (TS) through the pharynx or gut show the tissue layers: the outer epidermis, the gastrodermis lining the gut, and the mesenchyme between them. In good sections you can sometimes see rhabdites, rod-shaped bodies in the epidermis that are thought to help produce mucus.
These are good practice for reading tissue sections generally. See our beginner’s guide to histology for how sections are made and stained.
Do planaria regenerate?
Yes, and many species do so remarkably well. A planarian cut into pieces can regrow complete animals from each piece. Depending on species, temperature and cut position, this often takes one to a few weeks. Regeneration depends on neoblasts, adult stem cells spread throughout the body, which is why planaria are widely studied in stem cell research.
Not every species regenerates equally well, and some regenerate only from certain body regions. If you try regeneration experiments, be aware that they involve cutting a living animal. A sharp, clean blade on a chilled specimen is the usual practice. Many hobbyists simply watch planaria that split naturally: some species reproduce asexually by pulling apart into two pieces.
Planaria and parasitic flatworms
Flatworms (phylum Platyhelminthes) include both free-living and parasitic groups. The parasitic ones are mainly:
- Flukes (trematodes), such as liver and blood flukes.
- Tapeworms (cestodes).
- Monogeneans, which mostly live on the skin and gills of fish.
Planaria are not parasites, and the common freshwater species are not known to pose a risk to people. Even so, wash your hands after handling pond water.
Planaria in aquariums
Planaria sometimes appear in home aquariums, especially when there’s plenty of uneaten food. Aquarists generally see them as a nuisance because some species may prey on small shrimp or eggs. Cutting back on feeding and removing leftover food usually reduces their numbers. Some fish are reported to eat them, but results vary and you can’t rely on fish to clear an infestation. For microscopists, an aquarium outbreak is a free source of specimens.
Tips and responsible handling
- Move planaria with a soft paintbrush or a wide-tipped plastic pipette. Cutting the tip of a disposable pipette at an angle makes the opening larger.
- Keep observation sessions short and the water cool.
- Don’t release lab or aquarium planaria into the wild. Return wild-caught animals to the place you found them, or keep them.
- For other small water animals worth a look, see our guide to viewing microorganisms under a microscope, and for wet-mount technique, how to make a wet mount slide without air bubbles.