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Phylum Platyhelminthes ‘Flatworms’

Phylum Platyhelminthes ‘Flatworms’. I. Phylum Platyhelminthes (platy = flat; helminthes = worm). A. Characteristics. dorsoventrally flattened unsegmented triploblastic – three tissue layers; it is the first time that we see mesoderm in animals show cephalization – “head region”

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Phylum Platyhelminthes ‘Flatworms’

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  1. Phylum Platyhelminthes ‘Flatworms’

  2. I. Phylum Platyhelminthes (platy = flat; helminthes = worm) A. Characteristics • dorsoventrally flattened • unsegmented • triploblastic – three tissue layers; it is the first time that we see mesoderm in animals • show cephalization – “head region” • have a primitive gut • have some organs present : protonephridia and flame cells for excretion and osmoregulation • no circulatory or respiratory systems • most are monoecious

  3. 9. only one opening to digestive system so are said to have two-way digestion 10. no body cavity –tissue layers are stacked one on top of the other – they have no coelom a. these type of animals are said to be acoelomates

  4. 11. many species are parasitic 12. bilateral symmetric (have bilateral symmetry) 13. do have a nerve net with collections of nerve fibers in the head called ganglia 14. some free living species have eyespots that can detect the difference between light and dark. These eyespots are called ocelli

  5. B. Class Turbellaria (turbulence set off by whipping cilia) • free-living worms (non-parasitic) • there are both fresh and marine species • total nearly 300 species

  6. 4. exist as predators and scavengers, but not parasites 5. have ciliated epidermal cells which allow these organisms to move 6. rhabdites are special cells on the ventral side of the worm that can swell and form a mucous protective slime over the worm 7. parenchyma – (literally means “filling”) – this is the space filling tissue of mesoderm origin forming the middle layers of the worm 8. digestive system ranges from simple in some species to complex in others 9. include the freshwater Planaria and the many beautiful marine species of flatworms that exist found in the oceans 10. also includes the terrestrial “land” planarian

  7. 11. the small structures off the head are called auricles and have sensory organs located in them 12. feed through a tube called the pharynx Auricles

  8. 13. Reproduction • asexually by simply binary fission (splitting) into two zooids • have great powers of regeneration c. sexually (they are monoecious), but cannot self-fertilize • Produce reciprocal sperm • copulatory organ is the penis • eggs are contained in a structure called a cocoon • larva hatch as free-living forms called Muller’s larva

  9. Mating Planaria Muller’s Larva

  10. C. Class Monogenea 1. includes the monogenetic flukes that have but one generation in their life cycle D. Class Trematoda 1. class of the greatest number of flukes with 8,000 species

  11. 2. they attach by their mouths and also a structure at their posterior end called an acetabulum (see diagram in zoology coloring book ) 3. intermediate host is a snail in almost every case; many also have a fish as an intermediate host 4. these flukes live in the bile ducts and liver of their host 5. Some species of flukes are responsible for frog abnormalities in northern ponds of the U.S.

  12. E. Class Cestoidea • Cestodes are the tapeworms • they are the most highly specialized of the Platyhelminthes • there are 3500 species and all are endoparasites (living inside their hosts) • they have lost their digestive systems and are thus said to be degenerative parasites • since they cannot live outside their host, they are said to be obligate parasites 6. they lack a mouth, anus, or any digestive structures

  13. 7. cestodes are monoecious 8. the head of a tapeworm is called a scolex

  14. 9. each individual body segment is called a proglottid • each proglottid is a reproductive machine having both male and female reproductive organs in the same structure • younger, smaller proglottids form near the scolex c. older proglottids, further from the scolex, often are gravid (full of eggs) d. the strobila is the main part of the body consisting of many proglottids

  15. 10. Taenia solium is the pork tapeworm • can be up to 10 to 12 meters long • if the larva burrow into the muscle tissue they are called a cysticercus • if the cysticercus burrows into the brain, it can be fatal 11. life cycle of the beef tapeworm Taeniarhynchus saginatus

  16. adult tapeworm (3 meters in small intestine) eggs released in feces infected meat ingested Onchosphere larva cyticercus (bladder worm burrowed in muscle) Cattle graze on contaminated grass Larva cycles through the intestine, then into the blood, then burrows into the muscle tissue

  17. IV. Phylum Nemertea • known as the ribbon worms or also the proboscis worms • Characteristics: • Triploblastic • acoelomates • bilateral symmetric • complete digestive tract with both mouth and anus…one-way digestion • have protonephridia for excretion • have a closed circulatory system

  18. 7. have a food gathering device called a proboscis than can be extruded and act somewhat like an elephant’s trunk to capture food a. the proboscis is protected within a sheath called a rhynchocoel when not extruded proboscis

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