PLATYHELMINTHES Eumetazoa Bilateria Phylum Platyhelminthes flatworms marine freshwater

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. S 4 -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. S 4 -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. S 5 -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. S 5 -2) some hermaphroditic, others 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. S 6 -3 heart: muscular blood vessels dorsal, ventral blood vessel; Fig. S 6 -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