Annelida and Platyhelminthes

Review 1. Which morphological features differentiate the members of the phylum Annelida from nematodes and platyhelminthes? Platyhelminthes are worms with flat bodies (flatworms), whereas nematodes are worms with cylindrical but not segmented bodies (roundworms). Annelids are cylindrical worms with segmented bodies (they are metameric). 2. What is the key innovation in annelids? The key innovation found in the species of the phylum Annelida is the coelom, an internal body cavity completely covered by the mesoderm, a feature also present in arthropods, molluscs, echinoderms and chordates. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partially covered by the mesoderm). 3. What type of circulatory system is present in annelids? In organisms of the phylum Annelida, the circulatory system is closed, meaning that blood circulation takes place only within specialized vessels. 4. How can the presence, location and function of muscle tissue in organisms of the phylum Annelida be explained? These organisms contain a longitudinal muscle layer under the epidermis and, internally juxtaposed and perpendicular to it, another circular (radial to the axis) muscle layer. The circular muscle layer has the function of elongating the body while the longitudinal one shortens it. By alternating contractions, both promote movement. 5. How can the excretory system of annelids be described? In each segment (metamere) of the organism, a pair of complete excretory structures called the metanephridium exists. The metanephridium has an extremity, the nephrostoma, which collects waste from the coelom, filtering it and causing reabsorption along its extension (similar to human nephron tubules). The material to be excreted goes out through a pore, the nephridiopore, which opens to the surface of the body. 6. What is the clitellum of earthworms and where it is located? The clitellum is a special region of the annelid made up of rings (metameres) with a reproductive function. It can be found in the anterior portion of the animal and is distinguished by a lighter color in comparison to the normal color of the other segments. The main features of annelids. How can they be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction? 7. What are the best known representatives of the Platyhelminth phylum? The most well-known representatives of platyhelminthes are worms that cause human diseases, such as taenia and schistosome. Planaria, since it has been extensively studied in Biology, is also well-known. Examples of representative species: earthworms, leeches, lugworms. Basic morphology: cylindrical body, segmented (metameric). Type of symmetry: bilateral. Germ layers and coelom: coelomates. Digestive system: complete. Respiratory system: cutaneous or branchial. Circulatory system: closed, with hemoglobin. Excretory system: a pair of metanephridia in each metamere. Nervous system: neural cords, a pair of ganglia per metamere, anterior concentration of neurons (primitive brain). Types of reproduction: sexual.. 8. 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). 9. What is cephalization? Cephalization is the evolutionary tendency for concentration of nervous control in central structures in which neurons are grouped (including the brain and ganglia formations). Evolutionarily, the cephalization process begins with the appearance of ganglia (groups of neurons) in platyhelminthes and reaches peak in vertebrates, animals with a skull to protect a well-developed brain. 10. Into which classes are platyhelminthes divided? How can these classes be described and what are some representative species in 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 planaria (Dugesia tigrina). Trematodes are parasites, which live inside a host,. The schistosome (Schistosoma mansoni) that causes schistosomiasis is an example of one. Cestodes are also parasites. They have no digestive tract and their cells are nourished through the absorption of nutrients from their host. Their most well-known representative species are beef and pork taenia (Taenia saginata and Taenia solium), which are human parasites.