Introduction to Phylum Platyhelminthes

1. Introduction to Phylum Platyhelminthes Flatworms

2. Phylum Platyhelminthes • There are hundreds of species of flatworms that have been identified to date. • They are assigned to phylum Platyhelminthes. • Prefix platy means “flat” • Suffix helminth means “wormlike”

3. Phylum Platyhelminthes • Flatworms have a solid body construction • No body cavity between their gut and skin • Animals with no body cavity between their gut and body wall are termed acoelomate. • Acoelomate animals have a limited amount of organ development and are considered simple animals.

5. Phylum Platyhelminthes • When they evolved, flatworms were the first animals to have a middle embryonic cell layer, or mesoderm. • Thus, they (and higher phyla) have three layers in their embryo.

6. Phylum Platyhelminthes • As you recall, mesoderm makes possible the formation of muscle and other connective tissues, blood vessels, and organs such as the testes and ovaries.

7. Phylum Platyhelminthes • As a result of their mesoderm, flatworms possess a fair amount of muscle tissue and a reproductive system comparable to that of higher animals. • They do not however, have any blood vessels or circulatory system. • Materials are circulated through their body by diffusion. • Other systems flatworms do not include a respiratory system. Oxygen and carbon dioxide are exchanged across their skin by diffusion.

8. Phylum Platyhelminthes • Flatworms were the first animals to be bilaterally symmetrical. • They were also the first animals to develop a distinct head region containing sensory organs. • Animals that have ahead containing a concentration of nerve cells are said to be cephalized. • Cephalization enabled them to adapt to their surroundings more successfully. • Note: Zoologists credit them with the first eyespots

9. Phylum Platyhelminthes • Having no coelom nor circulatory system, flatworms must be thin enough so that dissolved substances, such as oxygen and carbon dioxide, can pass through their solid body by diffusion. • A thin body shortens the distance these substances must travel to reach each cell. • To help shorten the distance food substances must travel, the gut in a flatworm is highly branched.

10. Phylum Platyhelminthes • Portions of it run close to practically all of the flatworm’s tissues, giving each cell access to food molecules. • The gut has only one opening – the mouth – through which food enters and undigested food exits. • Thus, like the animals we studied before flatworms, the digestive system is primitive.

11. Phylum Platyhelminthes • There are three main classes in phylum Platyhelminthes: • The latter two classes consist of flatworms that live as parasites. • Turbellaria • Trematoda • Cestoda

12. Phylum Platyhelminthes • Trematoda and Cestodaare parasites. • This means that they live in and obtain their nutrition from a host organism • They cause untold misery and bring early death to immense numbers of humans, especially in the poorer parts of the world.

13. Class Turbellaria (Planarians) • Flatworms of the class Turbellaria are described as independent free-living. • This means that they move about and feed on their own. • Depending on the species, turbellarians can be found living in aquatic environments, damp soil, or fresh water. Dugesiatigrina Tricladida

14. Class Turbellaria (Planarians) • The most commonly studied member of class Turbellaria is the small freshwater flatworm planarians. • Several species of this flatworm have been identified. • They can be found under rocks and logs in streams and ponds. • They live partly as scavengers and partly as predators. • They grow to 20mmor less.

15. Class Trematoda (Flukes) • Flatworms of the class Trematoda are known as flukes. • They are all parasitic and may be very damaging to their hosts. • The Asian liver fluke is very common in many parts of Asia and is responsible for a great deal of sickness, disability and death. • People become infected by eating raw or improperly cooked meat carrying these fluke larvae.

16. Class Trematoda (Flukes) • Other species of flukes invade the lungs, spleen, intestines, or blood. • Schistosomes, or blood flukes, infects at least three hundred million humans in tropical regions. • These tiny worms drill through a person’s skin, enter the blood stream, and lay hundred of eggs. • The eggs can damage or block small blood vessels and often cause internal bleeding in the bladder and intestine. • Schistosomiasis kills approximately 600 thousand people in Asia, Africa, Latin America, and the Middle East.

17. Class Cestoda (Tapeworms) • Flatworms of the class Cestoda are known as tapeworms because of their shape. • These parasitic flatworms infect nearly almost all vertebrates. • The adult stage lives in the intestine of its host. • It depends upon the host not only for food but for the digestion process as well, as it lacks a digestive system.

18. Class Cestoda (Tapeworms) • The head of a tapeworm is known as a scolex and is equipped with one or more kinds of attachment organs, depending upon the species. • The segments making up the rest of the body are called proglottids.

19. Class Cestoda (Tapeworms) • These structures contain both male and female reproductive organs and produce packets of fertilized eggs. • As a tapeworm grows, it adds proglottids just behind the neck, pushing the older proglottids toward the end. The proglottids break off and pass out of the intestine with the feces.

21. Class Cestoda (Tapeworms) • Tapeworms that can infect humans include the beef tapeworm and pork tapeworm. • People become infected when they eat undercooked meat containing the larvae of these worms.

22. Venn Diagram Flatworms Cnidarians Sponges Cnidarians Endoderm Ectoderm Hermaphrodite Regeneration Gastrovascular cavity Bilateral Mesoderm: Acoelomate Organ Organ system: reproductive Ganglia: eyespot Radial symmetry Mesoglea No organs or organ systems Polyp/medusa Tentacles/stinging cells