Chapter 5 PHYLUM PLATYHELMINTHES
Phylum Platyhelminthes (Flat worms) One class is free living (Turbellaria) and most are parasitic, some commensals Tribloblastic bilaterally symmetrical dorsoventrally flattened Digestive system lacking or with muscular pharynx blind succular gut-no anus, single opining serve as mouth and anus Gas exchange by diffusion through the skin (tegument) that has a large surface area 2
excretory system protonephridial excretory system (ciliated flame cells or uniflagellated solenocytes) for nitrogenous waste (ammonia, urea and amino acids) elimination as well as osmoregulation (water and ion balance).
Nervous system cephalization; Nervous system with a cerebral ganglion (brain) and longitudinal and transverse nerve trunks;
Platyhelminths: Reproductive Systems Mostly monoecious with highly elaborated reproductive systems and complex life cycles that may involve several larval stages; Mostly cross fertilization. Male reproductive tract: one or more testes, one vas deferens or several vas efferentia that lead to vas deferens, seminal vesicle, ejaculatory canal with surrounding prostate glands, cirrus (penis) equipped with cirrus pouch (penis sheath) and opens into a cirrus atrium or male opening. 5
Female reproductive tract: One or two ovaries, oviduct/s, ootype with surrounding Mehlis glands, vitelline (yolk) glands, seminal receptacle, uterus, vagina that opens into cirrus atrium or female gonopore.
Male & Female Reproductive Systems 7
Classification of Platyhelminths I Classified into 4 classes: Turbellaria, Cestoda, Monogenea & Trematoda. 1. Class Turbellaria: Mostly free living benthic or planktonic in marine habitat Some in freshwater and moist terrestrial habitats; Eg. Dugesia, Planaria. --- Cellular epidermis multiciliated especially the ventral surface, secrete mucus from rhabdites; --- Locomotion by looping involving pedal waves resulting from circular (outer) and longitudinal(inner) muscles as well as epidermal cilia and especially duo-glands (adhesive and releaser glands) 8
Classes of Platyhelminths Class IV : Monogenea 9
Outercovering of Turbellaria & Trematodaa C.S. in Turbellarian worm showing ciliated cellular epidermis Syncytial Tegument of Trematodes 10
Classification of Platyhelminths III Class Turbellaria (Continued) ---Reproduction: -- Asexual: Body fragmentation & regeneration of lost parts. -- Sexual: worms are monoecious with complex and efficient male and female reproductive systems; mostly cross internal fertilization through cross reciprocal copulation or hypodermic impregnation or copulation; Two plans -- Fertilized egg released---juvenile stage (miniature turbellarian) in capsule---adult. -- Fertilized egg released---muller s larva (free swimming, ciliated microscopic and short lived)---adult. 11
Classification of Platyhelminths 2. Class Cestoda (Tapeworms): exclusively parasitic Adults are enteric parasites (in digestive tract of vertebrates including human) Larvae are tissue parasites of invertebrate & vertebrate intermediate hosts. Body regions: rostellated or unrostellated scolex with suckers, neck - No digestive tract strobila made of immature, mature and gravid proglittids (segments) --syncytial tegument with microthrices to increase surface area 12X for effective absorption of nutrients Monoecious proglottids; THUS DESCRIBED AS POLYZOOIC. 12
Adult Morphology Adults morphology, acetabulate scolex, bothriate scolex. 13
Adult Cestode and segment structure
Classification of Platyhelminths V Class Cestoda (Continued) Order Cyclophyllidea Two Life cycle patterns: --Taeniid Plan: Fertilized eggs containing hexacanth (onchosphere) come out of gravid proglottids one of different kinds of larval stages (cysticercoids, cysticercous, hydatid cyst) form in tissues of intermediate host Adult in Final (definitive) host after ingested none cocked muscles Examples: Taenia saginata, T. solium Echinococcus granulosus 15
Taenia L.C. 16
Echinococcus granulosus L.C. 17
Cestodes -- Pseudophyllidean Plan: Pseudophyllidean suckers, 2 groove like suckers Fertilized eggs containing hexacanth comes out of gravid proglottids eggs embryonate in water forming ciliated coracidium and hatch in water coracidium ingested by first intermediate host (Cyclops crustacean) where procercoid larvae develop Crustacean ingested by second intermediate host (fish) where pleurocercoid larvae develop in muscles Fish eaten by definitive host where adults develop in intestine (fish eating carnivores as cats and human) Family Diphyllobothridae; the largest of all tape worm Eg. Diphyllobothrium latum (fish tape worm), 15 meter long
Diphyllobothrium latum L.C. 19
Classification of Platyhelminths VI 3. Class Monogenea Mostly ectopoarasitic on marine and freshwater fishes & amphibians and reptiles Possess a mouth opining and blind ended digestive tract. Ingest host tissue and blood through the mouth With oral sucker and large opisthaptor (posterior attachment organ with hooks and suckers) Syncytial tegument with many folds to increase surface area. Incomplete digestive system Life cycle: Direct (without intermediate host): Fertilized egg released from host---onchomiracidium (larval stage)---attaches to host and develops into adult. Example: Gyrodactylus (on gills of fish) 20
Polystoma Anatomy Attachment through Opisthaber 21
Classification of Platyhelminths VI 4. Class Trematoda (Flukes): All parasitic mostly in various internal organs of vertebrates including humans syncytial tegument with many folds to increase surface area Oral and ventral suckers; Incomplete digestive system (mouthpharynx- blind caecum or caeca) Monoecious except for the schistosomes (which are dioecious) Life cycle: mostly with at least 2 hosts (Digenea) with snails as the first or only intermediate host, many larval stages before parasite reaches adult stage: Ferilized eggs ciliated miracidum ( in water or gut of snail intermediate host) mother and daughter sporocyst and/or redia (polyembryony in snail intermediate host) cercaria (goes out of snail) metacercaria (in second intermediate host if present) dult Examples: Schistosoma, Fasciola. 22
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Fasciola hepatica Life Cycle 24