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