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Genome, , For Bsc 1st sem, ZOOH, , Phone- 8972178094, , Classification of Phylum Platyhelminthes, Definition of Phylum Platyhelminthes:, Triploblastic, acoelomate, un-segmented and bilaterally symmetrical metazoans,, without anus, circulatory system, but with a mouth, protonephridial system,, parenchyma tissues within the space between the body wall and the gut, and spiral, cleavage development., Characteristic Features of Phylum Platyhelminthes:, 1. Phylum Platyhelminthes are triploblastic, acoelomate (without a body cavity) and, bilaterally symmetrical animals., 2. Body is soft, un-segmented and dorsoventrally flattened., 3. Metameric segmentation and skeletal structures, in any form, are absent., Pseudometamerism is seen in some members (e.g., Eucestoda)., 4. Anterior end of the body is differentiated into a head., 5. The body is covered by epidermis which is soft, syncytial and ciliated (e.g.,, Turbellaria) or in rest classes (e.g., Trematoda, Monogenea and Cestoidea) there are a, syncytial, non-ciliated body covering, called tegument., 6. The parenchyma tissue, a mesenchymal layer which occupies the entire space, between the outer body wall and the endoderm of the gut, develops from both, ectodermal and entomesodermal layer of the embryo., 7. The mouth of Phylum Platyhelminthes is the single opening of the digestive tract, and the anus is absent when the digestive canal is present. The digestive system is, totally absent in Cestoidea and Acoela, and digestive canal is branched in, turbellarians. So the digestive system is incomplete when present., 8. Locomotion of Phylum Platyhelminthes is effected by the use of cilia rather than, muscles., 9. There are no respiratory and circulatory systems. Haemoglobin is present in a very, few species (e.g., Derostoma, Syndesmis, Telorchis, etc.)., 10. The nervous system is most primitive type and is formed by longitudinal nerve, cords with a pair of cerebral ganglion or brain., 11. The excretory system consists of protonephridia with flame cells or bulbs.

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12. Reproductive system is generally hermaphroditic but the digenean flukes are, gonochoristic (separate sexes)., 13. Fertilization internal., 14. Cleavage spiral, and determinate in some species., 15. Development may be direct or indirect., 16. Flatworms are either mostly free-living (Turbellarians) or are internal or external, parasites (Trematodes, cestodes and monogeneans)., 17. Parthenogenesis and polyembryony usually occur in trematodes and tapeworms., The scheme of classification is based on partly, outlined by Ruppert and Barnes, (1994), taken from the book “Invertebrate Zoology”., Phylum- Platyhelminthes possess Four Classes…, Class- 1 Turbellaria, Class- 2 Trematoda, Class- 3 Monogenea, Class- 4 Cestoidea (= Cestoda), , Class 1. Turbellaria (Gk. turbella = a little string); Approx. 3000 species., Features:, 1. Mostly free-living, primarily aquatic and the great majority are marine and mostly, benthic; a few are terrestrial, some are commensals for parasites., 2. Generally small, flattened, bilaterally symmetrical with a low level of cephalization., 3. Un-segmented body., 4. Cilia scattered on epidermis., 5. Locomotion by cilia with muscular undulations., 6. Body surface of many species bears small, rod-like, hyaline rhabdites and related, rhabdoids—unique in turbellarians and the function is uncertain., 7. Presence of epidermal gland cells (Duo-gland, frontal gland, rhabdite gland) help, for adhesion, mucus secretion and other secretory functions., 8. Pigmented and some are brilliantly coloured., 9. Simple sac-like intestine with a simple or bulbous pharynx in small-size species,, and in large-size species the intestine is branched with a tubular pharynx.

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10. Body surface acting as a permanent respiratory surface., 11. Suckers absent., 12. Nervous system is a diffuse nerve-net with a cerebral ganglion, a primitive type, found in some forms and a distinct brain with 1-3 pairs of longitudinal nerve cords, found in advanced forms., 13. Pigment cup ocelli and statocysts are the sensory organs., 14. Excretory system is protonephridia which includes flame cells in most cases., 15. Mostly hermaphrodite., 16. Asexual reproduction in many species (e.g., freshwater and terrestrial, turbellarians)., 17. Spiral cleavage in many forms (e.g., in some acoels and polyclads),, 18. Generally development direct., 19. A free-swimming larval stage Muller’s larva is found in some forms (e.g., a few, entolecithal polyclads)., Examples:, Bdelloura (commensal on the book gills of horse-shoe crabs), Polycelis, Dugesia (previously called Planaria, common in ponds, lakes and streams), Euplanaria, Bipalium, and Geoplana (terrestrial triclads), Procotyla, Planaria (Northern Hemisphere),, Dugesia can differ from Planaria by the presence of eyes with pigment cups., , Class 2. Trematoda (Gk. trematodes = perforated):, Features:, 1. All are parasites., 2. Unsegmented, dorso-ventrally flattend body, hence trematodes are called “flukes”., 3. A syncytial, non-ciliated body covering or tegument present., 4. Rhabdites absent in epidermis., 5. One or more well developed suckers are present. Oral sucker for feeding and, ventral sucker (acetabulum) for attachment., 6. Digestive tract complete with two branches., 7. Mostly hermaphrodites., 8. No asexual reproduction.

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9. In most Cases the testes are two or many, but always single ovary., 10. Egg develops as a ciliated miracidium larva which infects the intermediate host, (molluscan)., 11. Most are endoparasites and have two or three hosts in the life cycle., Examples:, Fasciola, Schistosoma (Blood fluke), Aspidogaster, Stichocotyle, Cotylapsis., , Class 3. Monogenea (Gk. monos = single + genos = a race), Features:, 1. Ectoparasites of aquatic vertebrates (on fishes and also on amphibians and reptiles)., 2. Dorsoventrally flattened body., 3. Sucker is weak or absent., 4. Posterior end bears a large specialized adhesive disc, called haptor (= opisthaptor), which bears hooks and suckers, used for attachment. In addition there is an anterior, adhesive organ, called prohaptor, with adhesive glands and suckers., 5. Gut present but the mouth lacks a sucker., 6. Inconspicuous protonephridia include paired excretory pores situated, anterodorsally., 7. Single host in the life cycle., 8. There is no intermediate host and the one egg gives rise to only one adult worm,, hence the name “Monogenea”., 9. Ciliated Oncomiracidium larva in the life cycle which bears 3 bands of cilia and 1, to 2 pairs of eyes., There is no subclass or order under Monogenea., Examples: Gyrodactylus, Polystoma, Sphyranura, Dactylogyrus, Diplozoon., , Class 4. Cestoidea (= Cestoda) (Gk. kestos – a girdle, L. cestus = ribbon, eidos =, form; Approx. 3500 sp., Features:, 1. Gut parasites of the vertebrates., 2. Generally called tape-worms.

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3. Body covered by a non-ciliated syncytial tegument., 4. Body divided into 3 regions:, (i) Head or scolex usually with hooks and suckers for adhesion in the host’s gut,, (ii) The proliferative neck and, (iii) Strobila which consists of a large number of segments, called proglottids., 5. Mouth, digestive tract and sense organs absent., 6. Organs of attachment in the form of hooks and suckers are present., 7. Each segment excepting head and neck is provided with one or two sets of complex, sex organs., 8. Hermaphrodite., 9. Hooked embryos, called Oncosphere larva., 10. Complicated life cycle with one or more intermediate hosts., Examples: Taenia, Echinococcus, Echinobothrium (parasitize elasmobranch fish)