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PLATYHELMINTHES • Eumetazoa: Bilateria: Phylum Platyhelminthes (flatworms) • marine, freshwater, terrestrial PLATYHELMINTHES • • • • • either Protostomia or Lophotrochozoa triploblastic: 3 germ layers ectoderm: epidermis, nervous tissue mesoderm: true muscle tissue endoderm: gastrodermis lining gastrovascular cavity • acoelomate PLATYHELMINTHES • lack some organ systems (circulatory, respiratory) • diffusion: reason why small and flat • have others (muscular, nervous, digestive, excretory, reproductive) • nervous system (Fig. 49.2): eye spots, rudimentary brain, nerve cords • cephalization (head); Fig. 33.10, Fig. S4-2 PLATYHELMINTHES • hermaphroditic: male and female reprod. systems in same individual • adaptive advantage: every individual potential mate • digestive system: mouth, pharynx, gastrovascular cavity • one way gut; no anus • invagination: increases surface area PLATYHELMINTHES • excretory system: protonephridia (Fig. 44.11); proto = first, nephrus = kidney • branching network of tubules • flame-bulb cell with cilia draws mostly water, some waste into tubules • exit via nephridiopores PLATYHELMINTHES • free-living (non-parasitic) ancestral life history • many parasitic in vertebrate gut • suckers/hooks; Fig. S4-3 • reduce/lose some ancestral characters • tapeworms: no mouth/gastrovascular cavity/excretory structures; diffusion • mostly reproductive organs • Fig. 33.12 MALFORMED FROGS • mystery of the malformed frogs • August 1995: school kids in MN, field trip to a pond; amphibian malformation MALFORMED FROGS • most states, range of deformities • hypothesis: endocrine disrupting chemicals • disrupt development by disrupting hormones MALFORMED FROGS • alternative: trematode hypothesis • trematodes (= flukes) • Ribeiroia life cycle: encysts in limb buds MALFORMED FROGS • can cause deformities, but why now? • human-caused ecosystem disruption • nutrient runoff: organic pollution (fertilizer, manure, sewage) • increased snail population (a key host) • modified hypothesis: nutrient-trematode NEMATODA • Eumetazoa: Bilateria: Phylum Nematoda (roundworms) NEMATODA • Protostomia or Ecdysozoa • pseudocoelom • complete digestive tract (2 openings) hangs free in pseudocoelom; Fig. S5-1 • cylindrical, unsegmented body • cuticle: tough, thick outermost layer around epidermis; ecdysis NEMATODA • hydrostatic skeleton: fluid in pseudocoelom; high pressure • nervous system: anterior nerve ring, dorsal/ventral nerve cords • no circulatory/respiratory systems; diffusion • muscular system: longitudinal muscle • locomotion: contraction; thrashing NEMATODA • • • • only sexual reproduction (Fig. S5-2) some hermaphroditic, others dioecious dioecious: separate male, female reprod. system fills most of organism NEMATODA • • • • free-living (soil); parasitic uniform structurally, diverse ecologically species differ in specific habitat probably over one million species: speciose • 1 sq m of marine mud: 4.5 million individuals!; very successful • much to be learned NEMATODA • • • • • Caenorhabditis elegans (C. elegans) model organism free-living soil nematode; easy to raise transparent; life cycle: 3½ days every cell mapped, fate known; Fig. 47.22 C. elegans • • • • entire genetic sequence known huge medical applications humans, nematodes both bilaterians evolutionary medicine: studying other organisms for insight into humans ANNELIDA • Eumetazoa: Bilateria: Phylum Annelida (segmented worms) • Protostomia or Lophotrochozoa • coelomate • segmentation: repeated muscular, nervous, excretory elements divided by septae • annelida = little rings • Fig. 33.22 ANNELIDA • • • • • hydrostatic skeleton: fluid in coelom circulatory system: closed; Fig. S6-3 heart: muscular blood vessels dorsal, ventral blood vessel; Fig. S6-1 capillaries: small branches of blood vessels ANNELIDA • excretory system: metanephridia (Fig. 44.12) • fluid enters nephrostome, passes through coiled tubule • useful nutrients return to circ. system, wastes excreted via nephridiopore ANNELIDA • nervous system: ventral nerve cord with ganglia (swellings); brain • muscular system: longitudinal and circular muscles • locomotion: muscular contraction works against hydrostatic skeleton of coelom • setae (bristles) • digestive system: subdivided and more specialized • hermaphroditic or dioecious ANNELIDA • leeches • specialized as predators or blood feeders • don’t confuse with trematode • no obvious head; no septae or setae • suckers for temporary attachment • large digestive system for blood • secrete hirudin: anticoagulant • other secretions; osteoarthritis
