Reading: Flatworms

This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.

  • Phylum: Platyhelminthes (Flatworms)
    • Class: Turbellaria (planarians)
    • Class: Trematoda (Flukes)
    • Class: Cestoda (Tapeworms)


Flatworms are flattened and have bilateral symmetry.

They are triploblastic (have 3 embryonic tissue layers: ectoderm, mesoderm, and endoderm) and therefore have organ-level of organization. There is no body cavity, so they are acoelomate.

Flatworms have a gastrovascular cavity with one opening (a sac-like gut).

Free-living Species

Example: Dugesia—a freshwater planarian

Planarians have a branching sac-like gut (one opening).

The main function of the excretory system is for water regulation. It consists of two structures called protonephridia. Each protonephridium contains flame cells that move excess water into tubes that open to the outside.

Planarians have a head region with sense organs. The nervous system of Dugesia is somewhat more complex than the nerve net of Cnidarians. It consists of a brain and nerve cords arranged in a ladder-like configuration.

Planarians have ocelli (eyespots) allow the presence and intensity of light to be determined. These structures are covered but have an opening to one side and forward. They can tell the direction of light because shadows fall on some of the receptor cells while others are illuminated. They move away from light.

Planarians are hermaphroditic, that is, they contain both male and female sex organs. They can reproduce asexually simply by pinching in half; each half grows a new half.

Movement is accomplished by the use of cilia and also by muscular contractions.


Members of this group are primarily parasites (feed on a host species).

Parasitic forms lack cephalization.

The reproductive cycle typically involves two host species, a primary host and a secondary (or intermediate) host. Adults live in the primary host and larvae develop in the secondary host. The life cycle often alternates between sexual and asexual reproduction.

Liver flukes are found in vertebrate livers.

Nearly half of people in the tropics have blood flukes. Schistosomiasis is a blood fluke that afflicts 200 million people in the world. The secondary host is a snail.

Life cycle of trematodes: eggs hatch in water, larvae enters snail, asexual reproduction in snail, larvae enter water, larvae enter human beings, male and female copulate (sexual reproduction), eggs pass out in human feces, and the cycle repeats.


  1. Place a living planarian on a watch glass and observe its movements under a dissecting microscope. Look for the eyespots, auricles, gastrovascular cavity, and pharynx.
  2. Planarians cannot see images but they can tell the direction of light with their eyespots. Cover 1/2 of the watch glass with aluminum foil. Does the planarian favor the light area or the dark area?
  3. View a slide of a preserved planarian and note the eyespots, auricles, gastrovascular cavity, and pharynx.
Left: Planarian anterior end X 40. Middle: Planaria digestive tract mid section X 40. Right: Planarian c.s. X 40.

Figure 1. Left: Planarian anterior end X 40. Middle: Planarian digestive tract mid section X 40. Right: Planarian c.s. X 40.

Liver Flukes

Observe either a preserved liver fluke or a slide of a liver fluke using a dissecting microscope.

Figure 2. Sheep liver fluke (Fasciola hepatica) stainedLiver fluke (preserved)

Figure 2. Left: Sheep liver fluke (Fasciola hepatica) stained. Right: Liver fluke (preserved)


  1. View a preserved tapeworm (Taenia).
  2. View slides of Taenia. Locate the scolex. View a gravid (filled with eggs) proglottid.

Tapeworms live in the intestines of vertebrates.

They may reach 10 m in length (>30 feet). They have no digestive or nervous tissue. Attachment to the intestinal wall is by a scolex, a structure that contains hooks and suckers.

Figure 3. Taenia scolex X 40

Figure 3. Taenia scolex X 40

Taenia (preserved)

Figure 4. Taenia (preserved)

The segments (proglottids) each contain male and female reproductive organs. Eggs are fertilized from sperm, which often come from other proglottids of the same individual. After fertilization, other organs within the proglottid disintegrate and the proglottid becomes filled with eggs.

The intermediate hosts are usually pigs or cattle. They can become infected by drinking water contaminated with human feces.

Tapeworms can be passed to humans in undercooked meat, especially pork.

The photographs below show the scolex and proglottids at increasing distances from the scolex. Those segments closest to the scolex (on the left) are the smallest. Those furthest away (on the right) become filled with zygotes, break away, and pass out with the feces.

Figure 5. Left: Taenia pisiformis anterior end. Middle: Taenia pisiformis mid region. Right: Taenia pisiformis posterior end