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Flatworms - Study Them Here 1. What are the best known representatives of the platyhelminth phylum? The most popular representatives of the platyhelminthes are worms that cause human diseases, like taenia and schistosome. The planaria, since it is been extensively studied in Biology, is also well known. Image Diversity: platyhelminthes planaria taenia schistosome 2. What is the main external morphological feature that differentiates platyhelminthes from other worms (nematodes)? Platyhelminthes are also known as flatworms because they are worms with a flat body. This is the main external morphological feature that differentiates them from nematodes (roundworms). 3. How many germ layers originate the body of platyhelminthes? In relation to this characteristic how are these animals classified? Platyhelminthes are the first triploblastic animals (remember that cnidarians are diploblastic), i.e., they present three germ layers: ectoderm, mesoderm and endoderm. 4. What are the types of digestion and of digestive system of platyhelminthes? Flatworms have incomplete digestive systems and they present extracellular and intracellular complementary digestions. Image Diversity: platyhelminth digestive system 5. How are nutrients distributed by the digestive system in planarias? Planarias have single opening digestive system (incomplete) with ramifications that transport nutrients to all areas of the body. 6. How is gas exchange done in flatworms? Platyhelminthes exchange gases exclusively by diffusion through their body surface. This is only possible because all cells are localized relatively near to the exterior since gases diffuse cell by cell (the flat shape of these worms is a feature that allows this type of respiration). 7. Poriferans and cnidarians do not have excretory systems. Do platyhelminthes have an excretory system? Platyhelminthes have a primitive excretory system made of flame cells (also called solenocytes), excretory ducts and excretory pores. Image Diversity: platyhelminth excretory system 8. What is an example of freshwater flatworm? Due to that habitat what is the physiological problem that these animals must solve? Platyhelminthes of freshwater, like planarias, have an internal environment much more concentrated in solutes than the exterior and so they present a tendency to gain water. These organisms then need a drainage system to avoid cell death caused by excessive water. The problem is solved by the presence of protonephridia located along longitudinal channels in the animal body. Protonephridia have ciliated cells, the flame cells, that push water outside the body through excretory pores. 9. Is the nervous system in platyhelminthes more or less sophisticated than in cnidarians? What are the main neural structures found in flatworms? How is this neural organization important for the diversity of biological niches explored by species of the phylum? Platyhelminthes present a more sophisticated nervous system than cnidarians, as the first neural chords with ganglia (grouping of neurons) appear, a characteristic of the evolutionary process of increased nervous complexity. In platyhelminthes one can note the beginning of the cephalization process, with a concentration of neurons (nervous cells) in the anterior portion of the body and the appearance of photoreceptor cells in the ocelli. With the increased capacity of these animals to perceive and to interact with the surrounds due to the increased complexity of their neural complexity, it is possible to find platyhelminthes in a variety of environments, including the terrestrial, and with diverse ways of life, like those that are parasites and those that are free-living. Image Diversity: platyhelminth nervous system 10. What is cephalization? How does lateral symmetry favor cephalization? Cephalization is the evolutionary tendency of concentration of the nervous command in central structures in which there are grouping of neurons (i.e, brain and ganglia formation). Evolutionarily the cephalization process begins with the appearance of ganglia (group of neurons) in platyhelminthes and reaches an apex in vertebrates, animals with a cranial box to protect the well-developed brain. With lateral symmetry the body can be divided into lateral portions, superior, inferior, anterior and posterior. These portions must be integrated and controlled in some manner and this need stimulated the appearance of ganglial complexity and of beings with a head, a privileged extremity of the bilateral body where the nervous central command and important sensory organs are located. Image Diversity: cephalization 11. What is the type of reproduction, sexual or asexual, that occurs in platyhelminthes? Platyhelminthes may present sexual or asexual reproduction. 12. How can asexual reproduction in planarias be described? Planarias can divide themselves asexually by transversal bipartition due to the great regeneration capability of their tissues. When they attach to a substrate they can induce a constriction in their middle region and the body is then separated into two parts and each of these parts gives birth to a new individual as tissue regenerates. Image Diversity: reproduction in planaria 13. Are flatworms monoecious or dioecious? There are monoecious hermaphrodite flatworms, like planarias and taenias, and there are dioecious (having male and female individuals) species too, like schistosomes. 14. Is it possible for a hermaphrodite species to present cross-fecundation? There are hermaphrodite species of animals and plants that present cross-fecundation mainly due to the maturation of female and male structures at different periods. Cross fecundation occurs in planarias, hermaphrodites in which sexual fecundation takes place with male and female gametes from different individuals. These individuals approach their copulating structures and exchange gametes. 15. What is direct development? Is there a larval stage in planarias? Sexual reproduction with direct development is that in which there is not a larval stage in the embryonic development. When a larval stage exists it is said to be indirect development. In the sexual reproduction of planarias there is no larval stage. 16. Into which classes are platyhelminthes divided? How are these classes characterized and what are some representative beings of each of them? Platyhelminthes are divided into three classes: turbellarians (or Turbellaria), trematodes (or Trematoda) and cestodes (or Cestoda). Turbellarians are free-living platyhelminthes and their main representative is the planaria (Dugesia tigrina). Trematodes are parasites, they live inside a host and the schistosome (Schistosoma mansoni) that causes schistosomiasis is an example. Cestodes are parasites too, they do not have digestive tubes and their cells are nourished by absorption of nutrients from the host; their most popular representative are the beef and pork taenias (Taenia saginata and Taenia solium) that parasite humans. Image Diversity: turbellarians trematodes cestodes 17. What are the main human diseases caused by platyhelminthes? The main human diseases caused by platyhelminthes are schistosomiasis, tapeworm disease (cestodiasis) and cysticercosis. (Note:Diseases are studied in the “Diseases” division of this site.) 18. Platyhelminth identity card. How are platyhelminthes characterized according to examples of representing beings, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction? Examples of representing beings: planarias, schistosomes, taenias. Basic morphology: flat worm. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, acoelomates. Digestive system: incomplete. Respiratory system: nonexistent, respiration by diffusion. Circulatory system: nonexistent. Excretory system: protonephridia with flame cells. Nervous system: ganglial, beginning of cephalization. Types of reproduction: asexual and sexual. |