Planaria: adorable flatworms

Planarians are free-living aquatic flatworms that are staples of high school biology labs.  The species I was able to photograph, Dugesia tigrina, is fairly small, growing up to about an inch in length when stretched out.

A live brown speckled planarian {Dugesia tigrina} swimming in a dish full of water above a white background.  The light is coming from the left, and its shadow is subtly visible.  The pharynx (a tube the flatworm extends from its body for feeding) may be visible as a darkened tube in the middle of its body.  The planarian almost seems to be swimming as a sine wave. (Marc C. Perkins)
A live brown speckled planarian {Dugesia tigrina} swimming in a dish full of water above a white background. The light is coming from the left, and the worm's shadow is subtly visible. The pharynx (a tube the flatworm extends from its body for feeding) may be visible as a darkened tube in the middle of its body.

Planarians are utterly adorable.  Their heads have cute little eyespots (ocelli) that sense light and auricles (the triangular extensions) that reportedly sense water currents.  The eyespots lack lenses and a retina, so these cute little worms aren’t looking up at you and seeing your face, but they can detect the intensity and direction of light, allowing them to swim away from light (which is one of the easiest behaviors to observe in them; shine a light on them, and they’ll swim directly away from it). And when they move, they glide through the water with serpentine elegance.

A brown speckled planarian {Dugesia tigrina} swimming diagonally in a dish of water on a white background.  The planarian's eye spots (ocelli) and auricles are plainly visible in this closeup on its head. (Marc C. Perkins)
A brown speckled planarian {Dugesia tigrina} swimming diagonally in a dish of water on a white background. The planarian's eye spots (ocelli) and auricles are plainly visible in this closeup on its head.

The dark portions of the eye are not actually the photosensory nerves.  Instead, the dark portions are pigment-filled cells that partially surround the photosensory neurons, shading them from one side (thus allowing them to detect the direction of light without a lens, retina, or movable eye).

While many flatworms are parasitic, these planarians are not; they’re free-living omnivores that swim around in freshwater ponds nomming on whatever they can find.  In the lab we frequently feed them small pieces of liver or thymus.

A live brown speckled planarian {Dugesia tigrina} feeding on a small piece of Thymus.  The planarian's pharynx, a feeding tube that extends from its gastrovascular cavity (digestive tract) tract), is easily visible connecting the mid-section of the worm to the food.  These flatworms feed through their pharynx, which is located in the midsection of their body, not on their head.  The planarian's auricles and eye spots (ocelli) are planing visible. (Marc C. Perkins)
A live brown speckled planarian {Dugesia tigrina} feeding on a small piece of thymus. The planarian's pharynx, a feeding tube that extends from its gastrovascular cavity (digestive tract), is easily visible connecting the mid-section of the worm to the food.

The way flatworms feed is just awesome.  Instead of having a mouth at their head, they extend a tube (their pharynx) from the middle of their body and latch this tube onto their food.  They then “suck” the food up through this tube and into their digestive tract.

Speaking of guts, flatworms’ digestive tracts aren’t built like ours are: they have just a single opening that leads to and from their digestive track. This contrasts with our style of digestive tract, which has two openings: a mouth and an anus.  The planarian style of digestive tract is called a gastrovascular cavity, and it can be seen in the following image of a preserved planarian slide:

Planarian with a stained gut 2
A whole preserved planarian seen through a compound microscope after the gut has been filled with ink. The head is visible at the left, most notably the ocelli (eye spots) and auricles (triangular outgrowths from the head). In the center of the body is the pharynx, a long tubular structure that is extended from the body to feed. The pharynx connects to the gastrovascular cavity at the left end of the tube. The gastrovascular cavity extends throughout their body; in this individual it has been filled with black ink.

And yes, this does mean that digested food has only one way out: through the same opening that they used to get the food in.

Planarians are used in biology labs primarily thanks to their easy availability from biological supply houses ability to regrow tissues from traumatic injuries: when cut in half they can regrow the other half of their bodies. This is because while they can reproduce sexually using sperm and eggs, they can also reproduce asexually via fragmentation. Fragmentation is a reproduction mechanism wherein an organism literally pulls itself in half, with both halves growing into complete new organisms. This leads to the classic high school biology “experiment” wherein students cut flatworms in half and wait for them to regrow. We won’t be doing that here. But this picture of two flatworms swimming next to each other almost looks like it 🙂

The head of one brown speckled planarian {Dugesia tigrina} swimming up positioned next to the tail of a second planarian swimming down in a dish of water on a white background.  The planarian's eye spots (ocelli) and auricles are plainly visible in this closeup on its head.  Both head and tail are angled up and to the left. (Marc C. Perkins)
The head of one brown speckled planarian {Dugesia tigrina} swimming up positioned next to the tail of a second planarian swimming down in a dish of water on a white background.

I get live planarians each semester to show my biology classes, but sadly most students just give them a passing glance. Next time you get a chance to observe these cuties, put them in a dish of water, get a dissecting microscope and some liver, and plan to spend some time with them. They’re great fun!

A brown speckled planarian {Dugesia tigrina} turning in a circle on a white background.  The planarian's eye spots (ocelli) and auricles are plainly visible on its head.  The tail of the planarian is looped under the middle of the body, forming a circle. (Marc C. Perkins)
A brown speckled planarian {Dugesia tigrina} turning in a circle on a white background. The planarian's eye spots (ocelli) and auricles are plainly visible on its head.

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