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Phylum Nematoda Review
1. What are examples of nematodes? Ascaris, hookworm and filaria, all parasites of humans, are examples of nematodes (also known as roundworms). Phylum Nematoda Review - Image Diversity: roundworms ascaris hookworm 2. Are nematodes exclusively parasites? There are parasitic roundworms, including parasites of plants, but there are also free-living nematodes. 3. What is the typical morphological feature of nematodes that differentiates them from platyhelminthes? Nematodes are also known as roundworms. As the name indicates they are not flat like platyhelminthes. In evolutionary grounds with the nematodes the first complete digestive system appears, with mouth and anus, and the pseudocoelom is also a novelty. 4. What are the morphological similarities and differences between nematodes and annelids? Nematodes, like annelids, have a cylindrical elongated body. Annelids differentiate from nematodes by presenting a segmented body (body divided into metameres) and so they are called segmented worms. 5. Are nematodes diploblastic or triploblastic animals? Just like platyhelminthes, nematodes are triploblastics, i.e., they present three germ layers (ectoderm, mesoderm and endoderm). Phylum Nematoda Review - Image Diversity: germ layers 6. What is the main evolutionary innovation presented by nematodes? What is the advantage of that innovation? The main evolutionary innovation of nematodes is the complete digestive system, with two openings (mouth and anus). Since the ingestion and the defecation processes can occur in different extremities of the digestive tube, beings with a complete digestive system have the advantage of ingesting new food while residuals of already eaten food are still inside the body and not yet eliminated. Phylum Nematoda Review - Image Diversity: nematode digestive system 7. Compared to platyhelminthes which physiological problem have the cylindrical body of nematodes brought? How was that problem solved? The cylindrical shape of nematodes made impossible the respiration exclusively by simple diffusion among cells since there are tissues far from the exterior. This problem was solved by the presence of an inner cavity in the body filled with fluid, the pseudocoelom. The pseudocoelom has the function of distributing gases and nutrients to the body and to collect residuals, besides serving as a hydrostatic base to keep the worm shape. (For the fact that the pseudocoelom fluid and the pseudocoelom do not characterize a true circulatory system with blood and heart it is not said that in nematodes the respiration is cutaneous; it is considered that these animals still make respiration by diffusion). 8. How does the excretory system of nematodes work? The metabolic residuals of nematodes are collected by two longitudinal lateral excretory channels that open in one single excretory pore near the mouth. Phylum Nematoda Review - Image Diversity: nematode excretory system 9. How is the nervous system of nematodes organized? Where are the neural chords located in their body? Roundworms have a ganglial nervous system with an anterior neural ring representing (evolutionarily) a primitive cephalization. Nematodes have two main longitudinal ganglial chords that extend one dorsally and the other ventrally under the epidermis. There may also be nerves lateral to these main chords. The nervous system of a free-living nematode, Caenorhabditis elegans, has been well-studied in neurophysiological research and presents 302 neurons. The nematode C. Elegans was the organism used in the research on the genetic regulation of organogenesis and apoptosis whose researchers won the Nobel prize of Medicine in 2002 (Brenner, Horvitz and Sulston). Phylum Nematoda Review - Image Diversity: nematode nervous system 10. What is the type of reproduction that occurs in roundworms? What typical feature do nematode sperm cells have? Nematodes reproduce sexually. The nematode sperm cell does not have cilia nor flagella and they move by amoeboid movement forming pseudopods. 11. What are the main human diseases caused by roundworms? The main human diseases caused by nematodes are ascariasis, ancylostomiasis (hookworm infection) and filariasis (commonly known by its consequence, elephantiasis). 12. Nematode identity card. How are nematodes 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: ascaris, hookworms, filaria, pinworms. Basic morphology: cylindric (round) body, not segmented. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, pseudocoelomates. Digestive system: complete. Respiratory system: respiration by diffusion. Circulatory system: circulating fluid within the pseudocoelom. Excretory system: excretory channels and excretory pore. Nervous system: ventral and dorsal ganglial chord, primitive cephalization. Types of reproduction: sexual. |