Animal Body Plans: Body Cavities in Worms Microscope Lab

An online microscope lab for high school science on StudyScope. Students work through 4 real prepared microscope slides in order: Planaria (Transverse Section), Ascaris Female (Cross Section), Earthworm (Cross Section), and Tapeworm (Cross Section).

Students compare the body cavity in three worms cut straight across: a flatworm (planaria), a roundworm (Ascaris), and an earthworm. For each one, they work from the outside in, find the space between the gut and the body wall, and describe it before the labels name the body plan: no cavity (acoelomate), a cavity lined on one side (pseudocoelomate), or a true coelom lined on both sides (coelomate). Then they classify a tapeworm on their own and defend the call with evidence. Section titles name the worm but not its body plan. You drive the slides; students follow on their own screens and can pan and zoom. Have students sketch each cross section and keep a three-column table: "Body plan," "Example," and "Evidence I saw." About 40 minutes. To shorten it, skip the tapeworm and fill the table from the first three worms.

Grade level
High school
Time
40 minutes
Subject
Invertebrates
Slides
4
NGSS
HS-LS1-2, HS-LS4-1

NGSS alignment

Lab steps

  1. Worm 1: planaria

    Slide: Planaria (Transverse Section)

    • This is a planarian, a small flatworm from ponds and streams, cut straight across its body. Look at the whole section before zooming in. Describe its shape. Use the scale bar or the measure tool: about how wide is it, and how thick?
    • Zoom in on the middle. The branching open channel is the gut, where food is digested. Now look at the space between the gut and the outer skin. Is that space open and empty, or is it filled with something? Describe what you see.
    • Check the labels. The outer row of cells is the epidermis. The open channel is the gut. Everything between them is parenchyma, a loose but solid tissue. There is no open space around the gut, so planarians are called acoelomates, which means "without a body cavity." Sketch the section and label these three parts. Then start your table with this row.
    • Planarians have no lungs, gills, or blood vessels. Oxygen soaks in through the skin and passes from cell to cell. How does a flat body help every cell get enough oxygen? What would happen to the cells in the middle of a planarian as thick as your finger?

  2. Worm 2: Ascaris, a roundworm

    Slide: Ascaris Female (Cross Section)

    • Now a roundworm, Ascaris, cut straight across. It is a parasite that lives in the human intestine, and this one is a female. Look at the whole section. How is its shape different from the planarian? What seems to fill most of the inside?
    • Zoom in on the edge and work inward. Find a thick, smooth outer layer, then a band shaped like the teeth of a comb, then an open space, then a long tube lined with tall cells. Is there an open space between the outer wall and the tube? How is that different from the planarian?
    • Check the labels. The outer layer is the cuticle, a tough coat with no cells in it. The comb is a layer of muscle cells. The long tube is the intestine: one layer of tall cells, with no muscle of its own. The open space between the muscle and the organs is a body cavity. In a living worm it is full of fluid. Sketch the layers in order, from the outside in.
    • Check the labels in the middle. The two large ovals are the uteri, packed with eggs. The small round tubes around them are parts of the ovary. All of these organs sit in the body cavity. This cavity is lined with tissue only on the outer side, so it is called a pseudocoelom ("false body cavity"). Ascaris is a pseudocoelomate. Add this row to your table.
    • The fluid in the cavity is under pressure, a bit like the water in a water balloon. The worm's muscles pull against it. How could a fluid-filled cavity help a worm move? Why might a parasite that makes huge numbers of eggs need open space inside its body?

  3. Worm 3: earthworm

    Slide: Earthworm (Cross Section)

    • Now an earthworm, cut straight across. This slide is lightly stained, so look closely. Start with the whole section. Compare its outline and outer wall with Ascaris. Which part of the earthworm's wall looks thickest?
    • Zoom in on the wall. Under a thin outer band, you'll see thick pink bundles that look like rows of feathers. These are muscles. An earthworm has two muscle layers that pull in different directions. Predict: what could each layer do to the shape of the worm?
    • Check the labels. The thin outer band holds the cuticle, the skin, and a thin ring of circular muscle. The feathery bundles are longitudinal muscle, which runs the length of the worm. Circular muscle squeezes a segment long and thin; longitudinal muscle pulls it short and fat. Waves of the two let the worm crawl. Sketch the wall and label both muscle layers.
    • Check the label in the middle. Inside the muscle wall is the coelom, the body cavity. The gut should run through it, but on this slide it tore and is mostly missing. In a living earthworm, the coelom is lined with tissue on both sides: along the body wall and around the gut. That makes it a true coelom, and earthworms are coelomates. Add this row to your table.
    • In an earthworm, the gut has its own muscle. In Ascaris, the intestine is a single layer of cells with none. Which worm can squeeze food along its gut without moving its whole body? How does a cavity lined on both sides make that possible?

  4. Your call: tapeworm

    Slide: Tapeworm (Cross Section)

    • Your call. This is a tapeworm, a parasite that lives in a dog's intestine, cut across one segment of its body. Look at the whole section first. Describe its shape and what fills the inside.
    • Zoom in and hunt for a body cavity. Look at the two large round holes near the ends and the open spaces in the middle. Then search for a gut. Is the tapeworm an acoelomate, a pseudocoelomate, or a coelomate? Write your answer and two pieces of evidence before the next step.
    • Check the labels. The tapeworm is a flatworm like the planarian, and an acoelomate: solid parenchyma fills its body. The two round holes are excretory canals, tubes that carry wastes along the body. The spaces in the middle belong to the reproductive organs. There is no gut at all. A tapeworm soaks up food its host already digested, right through its outer covering, the tegument. Did your evidence hold up? Which clue was the hardest to read?
    • Finish your table with three rows: acoelomate, pseudocoelomate, and coelomate. For each one, write an example from today and one piece of evidence you saw on the slide. A planarian glides through ponds, and a tapeworm lives inside a dog. Why would biologists still put them in the same group?

Slides used in this lab

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