Phylum Platyhelminthes

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Phylum Platyhelminthes
(Flatworms)
20,000 species
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Characteristics

• Range from lt1mm to 25m
• Flattened shape (for gas exchange) and
  unsegmented
• Triploblastic (mesoderm called parenchyma)
• Acoelomate
• Bilaterally symmetrical

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Characteristics

• First phyla with organ system level
• Incomplete gut
• Somewhat cephalized (anterior ganglion and
  longitudinal nerve cords)
• Excretory structures (protonephridia)
• Hermaphroditic (complex structures)
• Three muscle types (longitudinal, circular,
  oblique)

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Class Turbellaria
3000 species
planaria
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Turbellaria

• Free-living (non-parasitic)
• Predators or scavengers
• Named for turbulence that beating cilia create in
  the water

cilia
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Turbellaria

• Ventral surface has gland cells that form mucus
  sheath, produce attachment chemical, and produce
  releaser chemical
• Lay down sheet of mucus as they move

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-Digestive cavity varies from simple to highly
branched (no circulatory system)-Pharynx tube
from mouth -Respiration through
diffusion
eyespot
pharynx
-Flame cells excrete liquid waste -Two simple
eyespots (ocelli) sense light
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• Reproduce asexually by transverse fission
  produce zooids (regeneration)
• Monoecious internal fertilization (exchange
  sperm) lay eggs

fission
egg
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Class Monogenea

• Flukes with only one generation
• External parasites (fish)

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Class Trematoda
The Flukes
schistosoma
8,000 species
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• Wide, flat shape
• Parasitic as adult
• Feed on host cells or fragments (blood, body
  fluids)
• Most have oral sucker and acetabulum to attach to
  host (two suckers)
• Outer layer called tegument (protects from hosts
  immune system)

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• Some require at least two different sources
  (example, larva in snail and adult in sheep)

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Important human parasite
(second only to malaria)
Sheep liver fluke
Schistosomes
from aquatic plants
-freshwater snail host
-enter through skin
Chinese liver fluke
from raw fish
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Class Cestoidea
The tapeworms
3,500 species
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Scolex

• Most highly specialized
• Endoparasites (digestive system intestines)
• Lack mouth and digestive tract attach to host by
  knob shaped scolex with suckers absorbs
  nutrients directly from host

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• Outer layer called cuticle
• Consists of repeating units called proglottids
• New proglottids form at neck, old ones (mature)
  reproduce and break free to release eggs
• Reproduction with other mature proglottids of
  same worm or another worm

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Also important human parasites

• Beef tapeworm pasture contaminated with feces
  cows graze and tapeworm enters bloodstream to
  muscles consumed by human attach to intestines

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• Pork tapeworm can release eggs were larva bore
  hole through intestine to bloodstream and encyst
  in tissue

fish tapeworm
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Phylum Gastrotricha
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Aschelminthes Roundworms
7 phyla
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Characteristics

• Psuedocoelom
• Most have complete tubular digestive tract
  alimentary canal
• Two body openings
• Some show eutely number of cells constant for
  animal and organs
• Bilateral symmetry
• Triploblastic

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Phylum Rotifera
2,000 species
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• Characteristic
• ciliated organ
• corona
• Free-living
• Marine and freshwater
• Pharynx contains mastax (jaws)
• Some reproduce through parthenogenesis

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Phylum Kinorhyncha
Burrow into sand with snout
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Phylum Nematomorpha
Horsehair worms
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Phylum Acanthocephala
Thorny headed worms
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Phylum Loricifera
most recently discovered
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Phylum Priapulida
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Phylum Nematoda
16,000 species
(maybe as many as 500,000)
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• Most parasites affect almost all species of
  plants and animals
• Feed on plants by sucking juices (fruit and
  cotton industry affected)
• Humans host about 50 species
• More than 1/3 human population suffers

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• 1 shovel soil contains over 1 million nematodes
  (5 billion in an acre)

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• Covered with tough cuticle
• and tapered at both ends
• Absence of circular muscles characteristic
  thrashing
• Excrete liquids through
• excretory pore

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• Most dioecious females usually larger
• Guinea worm males 1, females 2-4
• Cloaca- common chamber for digestive and
  reproductive tract
• Sperm stored in female body

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Ascaris
Intestinal roundworms
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• Largest nematodes- can grow over 1 foot
• Also infect pigs and horses
• Does not cause serious health problems unless
  many block intestine

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• Human eats veggies grown in infested soil (eggs)
• Eggs hatch in intestines, bore hole through wall
  and enter blood stream to lungs
• Larvae coughed up into mouth, swallowed and
  returned to intestine
• Develop into mature adults and reproduce
• Fertilized eggs leave host in feces and may be
  picked up by other hosts (eggs well protected by
  tough shells- up to 5 years)

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Hookworm
Bent anterior end
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• Attaches to intestine and sucks blood- can cause
  much damage as host often loses blood (anemia)
• Serious problem in warm, moist areas where people
  walk barefoot
• Larvae develop in soil and enter host through
  cracks in foot
• Life cycle like Ascaris

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Trichina
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• Smaller (1.5-4mm)
• Life cycle begins in pig (or other mammal)-
  reproduce sexually
• Fertilized eggs hatch in body and larvae
  deposited in lining of intestines
• Larvae get to blood and travel to all parts of
  body
• Worms lodge in muscle where they develop cysts
  and cause cramps
• Muscle tissue is eaten- 1 gram of pork can
  contain 3,000 cysts
• Heat of cooking kills trichina
• Trichinosis uncommon in US- farmers cook meat
  scraps fed to hogs

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Trichinosis
undercooked pork
cyst in muscle
bear meat
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Filarial worm
Filariasis (elephantiasis)
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Guinea worm
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Loa loa worm
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River blindness
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Phylum Annelida
"Little rings"
9,000 species
Segmented worms
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• Bodies divided into a series of segments- visible
  as rings on outside of body- separated by
  septa--- metameric
• Each segment has own excretory, nervous, and
  circulatory structures
• Flexible support and efficient locomotion

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Characteristics

• Most complex of all worm
• Triploblastic
• Bilateral symmetry
• True coelom
• More complex circulatory, respiratory, and
  nervous systems
• Closed circulatory system

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Characteristics

• Two sets of muscles- circular and longitudinal
• Paired epidermal setae (bristles)
• Ventral nerve cord

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Class Oligochaeta
Earthworms
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Mostly live in soils
(some aquatic and marine)
Vary in size- few cm to 11 ft
Most of 100-150 segments identical
except ends and 35-37 (clitellum)
Moves by anchoring setae on
posterior segments then
contracting circular muscles
in front
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Digestion

• Takes in soil through pharynx
• Soil moves down esophagus to crop (storage)
• Moves to gizzard- crushed by grinding
• Moves to intestine- absorbed into blood through
  folds in wall
• Solid waste out through anus
• Valuable to gardeners and farmers (break up soil,
  aerate, and add nutrients)

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Circulation- closed system

• Blood circulates through series of vessels (run
  length of body)
• Blood absorbs molecules and carries them through
  dorsal vessel to five pairs of hearts
• Hearts pump blood into main ventral blood vessel
  and smaller vessels carry blood to body

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Respiration and Excretion

• Take in O2 and give off CO2 by diffusion through
  skin
• Must remain moist (not too wet or dry)
• Eliminate liquid wastes through ciliated tubes
  called metanephridia (2 per segment)
• Increased by extensions of body surface called
  gills

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Nervous Control

• Can respond rapidly due to concentration of
  nerves called cerebral ganglion (anterior)
• Connected to rest of body by ventral nerve cord
• Ganglion connects each segment to cord
• No external eyes or ears- receptors in skin
  enable reactions (light, sound, chemical)
• Active mainly at night
• Do not respond to red light

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Reproduction

• Hermaphroditic
• Female structures anterior, male posterior
• 2 worms exchange sperm- mucus secretion from
  clitellum holds together
• Sperm stored in seminal receptacle until just
  before eggs laid
• Mucus case formed to push eggs and sperm through
  body and protect until hatching

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Class Hirudinea
Leeches
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• Fresh water
• Do not have setae
• Flat and tapered body with suckers on each end
• Feed on blood of other animals- attach by
  anterior suckers
• Three sharp jaws to pierce host skin
• Secrete substance that prevents hosts blood from
  clotting

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• Used medically to bleed patients

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Class Polychaeta
"many bristles"
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• Mostly marine
• Largest group
• Burrow into sea floor or live in reefs or tubes
• Carnivores or filter feeders
• Lateral extensions called parapodia with numerous
  setae

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• Remarkable regeneration
• Some produce epitokes- reproductive individuals
  different from nonreproductive individuals