P R E S E N T E D BY: N I C K KO R E E N

PLATYHELMINTHES

EVOLUTIONARY HISTORY

• Clade Bilateria- exhibit bilateral symmetry and triploblastic development. Vast majority of animals fall into this clade.

• Most recent common ancestor about 575 million years ago, first appeared during the Cambrian explosion.

• Clade Lophotrochozoa- name derived from a feature that some members have called a lophophore otherwise referred to as a crown of ciliated tentacles.

MORE EVOLUTIONARY PERSPECTIVE

• Lophotrochozoans include 18 phyla one of which is Platyhelminthes.• The most body form diversity is found within the

Lophotochozoan clade. Very few if any morphological features are shared amongst group members.

THE BIG PICTURE

UNIQUE CHARACTERISTICS

• Bilateral symmetry- this is a derived characteristic that indicates only one plane of symmetry.

• Central nervous system- along with sensory structures.• Acoelomates- No body cavity or organs for circulation.• Literal translation is flat worm, body shape is flattened

and thin, but can range in size from microscopic to 20 meters.

• Triploblastic development- all three germ layers (ecto, endo, and mesoderm). This is especially interesting because they lack a body cavity which means that all cells are placed wither next to the surrounding water or in their gut. This allows for gas exchange via diffusion and for osmotic balance to be maintained.

CLASS TURBELLARIA

• 3,000 species in 12 orders.• Marine, freshwater, and terrestrial.• Uniquely separated by their free-living ability (whirlpool).• Movement via cilia on mucus trail.• One opening (mouth), secret via the epidermis• Eye spots, but not image forming eyes, simply photoreceptors

and chemoreceptors.• Web of nerve fibers allow for fine muscular movement.

CLASS TREMATODA

• 9,000 species that are all parasitic.• Have one or two suckers that separate them from the

class Turbellaria.• Covered with tegument which allows the cell to lie

deep, with a dividing layer of muscle between the cytoplasm and the cell.

• Complex life cycle where the larval stages occupy a different host then the adult stage.

• Unlike the free living Turbellaria they have an enormous impact on humans via liver and blood flukes.

CLASS CESTODA

• 5,000 species all are endoparasites. There are two sub classes, cestodaria and eucestoda.

• Longer bodies with identifiable scolex (head with suckers and hooks).

• No cilia, but they have tegument covered in microvilli.

• Body has true segments each one acting as a reproductive unit.

ANATOMY AND PHYSIOLOGY

• Uniqueness amongst the classes, but bilateral symmetry, triploblastic tissue layers, and acoelomate are common features.

FEEDING/NUTRITION MODE

• Two of the classes are strictly parasites which means they are relying on at least one host if not more for nutrition. • For these species absorption of nutrients via the

epidermis is the uptake process. • Digestive system is not present so absorption is

right into the cells and secretion is also out of the epidermis.• Free-living worms do have a mouth and a feeding

tube which allows for consumption. These flat worms prey on smaller or dead animals but they can also be herbivores or ectoparasites.

THE LIFE CYCLE

• Amazingly complex- in particular the two completely parasitic classes. • Sexual and asexual reproduction possible • Flatworms are hermaphrodites and often cross

fertilize each other. But they can also reproduce asexually through transverse fission. • A small amount of flatworms can even reproduce

asexually via budding.

THE LIFE CYCLE: CLOSER LOOK

ECOLOGICAL CONSIDERATIONS

• Some free-living flat worms are very colorful and are in demand in the aquarium trade. Most are common laboratory animals, but in general these worms can handle pollution of aquatic habitats well and therefore there have been very few cases of humans negatively affecting their populations.

• Free-living flat worms can funnel nutrients to higher trophic levels as they are prey for high-level consumers.

• Some flat worms are important mutualistic and commensal symbionts.

• Flatworms regulate some populations by reducing the fitness of their hosts and sometimes causing death.

A FEW EXAMPLES

Planarian (bottom right) is a common ocean flatworm, liver and blood flukes along with tapeworms are other common examples of species within this phyla.

COOL STUFF

• There have been recorded tapeworms living in fish that were over 20 meters long.

• 42.1 million people were treated for the disease caused by blood flukes last year alone.

• There are less than 1,000 reported cases of tapeworm in the United States each year.

• Did you know… that planarian flatworms don’t age. The telomeres that act as a biological clocks at the end of chromosomes stay intact.

• Scientists can train flatworms to do simple tasks, such as finding water in a maze. When these flatworms are then cut in half, the new halves learn faster than the original

• If a flatworm is starved it is capable of shrinking to hatching size and when fed it has the ability to grow back to its original size

REFERENCES

• Campbell, A. 2001. "Turbellaria" (On-line), Animal Diversity Web. Accessed November 20, 2014 at http://animaldiversity.ummz.umich.edu/accounts/Turbellaria/

• Flatworm. (n.d.). Retrieved November 20, 2014, from http://www.ecospark.ca/changingcurrents/flatworm

• Karleskint, G., & Turner, R. (2006). Introduction to marine biology (2nd ed.). Southbank, Victoria: Thomson.

• Reece, J. (2011). Campbell biology Jane B. Reece ... [et al.]. (9th ed.). Boston: Benjamin Cummings :.

• Tapeworms = Cestoda. (n.d.). Retrieved November 20, 2014, from http://www.earthlife.net/inverts/cestoda.html

• TREMATODES. (n.d.). Retrieved November 20, 2014, from http://www.colorado.edu/eeb/amphibianparasites/Trematodes.html