Introduction to the Flatworms

Дата канвертавання24.04.2016
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Biology 120 Lab


Introduction to the Flatworms

Members of the phylum Platyhelminthes are commonly called flatworms. There are both parasitic (dependent on a host organism for survival) and free-living forms of this group. Flatworms, either parasitic or free-living forms, have both sexes on the same individual, a condition referred to as hermaphroditic. Another term that describes this condition is monoecious. Some parasitic species will self-fertilize their own eggs if another individual is not present.
This group does not have a body cavity (coelom) and is referred to as acoelomate. Flatworms are the first animal group to show all three tissue layers that we find in most animals (except Sponges and Cnidarians). This condition is referred to as triploblastic. These three embryonic tissues are: ectoderm, which forms the skin or epidermis and nervous tissue, mesoderm, which forms muscles and some organs, and endoderm, which forms the lining of the gastrovascular cavity or gut.

Introduction to the Roundworms

All roundworms, phylum Nematoda, are very similar in appearance: they have a cylindrical body with tapering ends. Most Nematodes are free-living, but some are important parasites of plants and animals. As for the flatworms, nematode development includes all three embryonic tissue layers. Roundworms have a fluid-filled body cavity, that is lined on the outside by tissue derived from mesoderm. Such a cavity is called a pseudocoelom.

Introduction to the Segmented Worms

Some worms are comprised of many segments. These animals belong to the phylum Annelida. Members of this phylum may be most familiar: the common earthworm, leech and nightcrawler belong to this group. There are three general types of segmented worms. One group is entirely marine (the polychaetes), while the other two groups are either terrestrial or live in freshwater. Annelids have a true coelom, a condition called coelomate. That is the body cavity is lined on the inside and outside by mesoderm derived tissue.


You will be looking at living specimens, preserved specimens for dissection, and preserved specimens for observation. Please handle all specimens with care. Follow the procedures below and document your findings on the lab report.

Free-living Flatworms

The free-living flatworm in the bowl at the front of the classroom is the common planarian Dugesia, which may be found under rocks in cold, clean streams. Observe the living planarian and note its general shape. What type of body symmetry does it possess? Does it have a head end? A head end with a concentration of nervous tissue is called cephalization.

Draw a sketch of a planarian from either the living specimens or a whole mount microscope slide. Are the anterior and posterior ends the same or different? Where would you expect to find the animal’s sensory receptors and ‘brain’? Label the head end on your sketch. Do you see any structures that may be sensory organs? Describe them.

Examine the planaria light demonstration and record your observations. Do planaria prefer the light or the shade? What do you think are the benefits afforded to planaria?

Examine the Planaria slide and draw/identify the following structures: Ocellus, Auricle, Anterior and posterior branches of intestine, Everted pharynx (proboscis).

Using a prepared slide (planaria digestive tract w.m. or planaria plain and digestive tract w.m.), sketch the digestive tract of a planarian. Is it simple like the cnidarians or more complex? How many openings to this tract do you see? What is similar and/or different about the gastrovascular cavity of the planarian compared with the hydra or jelly from last week? What might an advantage of the highly branched gastrovascular cavity be to flatworms?

Parasitic Flatworms

There are two types of parasitic flatworms: the tapeworm and the fluke. Parasites are organisms that spend some portion of their lives in close association with their host organism. This relationship weakens and can even kill the host. These organisms may live on the outside of the host (ectoparasites) or within the host (endoparasites). Tapeworms seem to specialize in the gut tracts of many kinds of vertebrates, while flukes are found in the blood stream or organs associated with the blood, like the liver. Look at the preserved specimens and slides of tapeworms and flukes. Notice the body shape of each. What do these animals use to anchor them in place on their hosts? Both of these parasitic worms have complex life cycles; learn about flatworm parasitic life cycles.

Sketch and label the tapeworm and fluke from the prepared whole mount slides (the tapeworm slide is labeled Taenia pisiformis and the fluke is Clonorchis sinensis. These are the species names for each animal. Where do these organisms attach to their host? Label these parts on the drawings.

Sketch and label the tapeworm (Taenia pisiformis) scolex.


The parasitic Ascaris lives in small intestine of humans and pigs, feeding on partially digested food. Its life cycle is shown in the preserved model. Note that females can be distinguished from males because they are larger and do not have a curved or hooked posterior end.
Draw and describe the Ascaris preserved specimen located on the counter with other specimens. Can you identify males from females?
Sketch a cross section of a nematode (Ascaris lumbricoides m. & f. c.s.). Find and label the muscles, uterus, intestine, pseudocoelom, dorsal and ventral nerve cords.

Draw and label the following nematodes: Enterobius vermicularis (pin worm), Nectar americanus, & Trichinella spiralis.


Be able to identify some representative members of Annelida from the segmented worm-demonstrations).

Watch an earthworm move. Based on its movement, predict what types of muscle annelid worms have.

Draw a cross section of the earthworm (Slides: Earthworm intestinal region c.s. & Earthworm posterior to clitellum c.s.). Identify the muscles in the drawing. What advantage might a segmented worm have over a non-segmented worm (e.g. Ascaris) have in terms of movement?

Finally identify (draw & label) few internal structures.

Dissection Instructions for Earthworm

1. Dissection should be done with care. Pin each end of the preserved roundworm to the bottom of a waxed dissecting pan..
2. Refer to the illustration of worm dissection (diagrams are clearer than the photographs). Identify the dorsal (back) side of your earthworm. It is darker than the ventral (belly) side of the worm. Place your earthworm dorsal side up in a dissecting pan. Pin the earthworm to the dissecting pan by inserting a pin in the first and last segments. Try to place pins as close to the end of the worm as possible to avoid so that you do not pucturing any important structures. Stretch the worm slightly as you add the second pin.
3. Next, identify the clitellum, a distinctive landmark. The clitellum is a section of the worm that is about 1 cm long, where it appears lighter colored and has a slightly larger circumference than the rest of the worm. It is found closer to the anterior (front) end of the worm. The end of the worm further from the clitellum is the posterior (rear) end. Most of the interesting structures are found at the anterior end of the earthworm. Posterior to the clitellum, all of the segments are essentially identical.
4. You must now carefully cut the skin of the earthworm and open it up to view internal structures without destroying them in the process. The best way to open the earthworm is with scissors. Insert the tip of the scissors about one inch posterior to the clitellum. Then lift up on the scissors to pull the skin away from the delicate structures below as you cut. Carefully cut forward from this point to the anterior end of the worm, keeping the scissors held up where they will not damage structures below. You may also cut posteriorly towards the pin holding the end of the worm in place.
5. Pin open the incision as you cut so that the body cavity is exposed. Place the pins at such an angle that they will not obstruct your view. You may need to gently use a dissecting probe to pry the skin away from the structures below.
6. Trace the digestive system from mouth to anus, paying special attention to the sections of the digestive tract that are modified to aid in digestion: pharynx, crop, gizzard, intestine.
7. Note the size (relative to other structures) of the seminal vesicles and seminal receptacles --sperm storage areas.
8. Finally, pull the intestine out of the way to reveal the ventral nerve cord.

Draw the dissected annelid and label some of the major organs you discovered

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