Biology 320
Invertebrate Zoology
Fall 2005
Chapter 10 – Phylum
Platyhelminthes
Phylum Platyhelminthes
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20,000 species of soft,
dorsoventrally compressed worms
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Mainly marine or freshwater
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Three major groups
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Class Turbellaria – free-living
flatworms
Class Trematoda – parasitic flukes
Class Cestoda – parasitic tapeworms
Most are acoelomate and also lack a
hemal system
Small size is an adaptation to living
in tight spaces, such as under rocks,
in body cavities, etc.
Class Turbellaria
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Two types, grouped
according to size
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Microturbellaria
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Microturbellaria
Macroturbellaria
The majority of turbellarians
at 4500 spp.
0.5mm – a few mm
Macroturbellaria
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Normally several cm
Largest lives in a lake in
Russia and can reach 60cm
in length
Body Wall
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Typically have one layer of ciliated epidermis
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Cilia may only be present on ventral surface
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Name means “whirlpool,” due to ciliary action
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Microvilli also present on epidermis
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Soft; lack a cuticle
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Often have a fibrous
endoskeleton containing
actin filaments
Epidermis is glandular;
secretes:
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Mucus
Adhesives
Rhabdoids
Rhabdoids
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Membrane-bound
Rod-shaped
Released onto epidermal
surface
Expand to form mucus
Rhabdite is the most
common type
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Adhesion facilitated
by
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Glands
Cilia
Muscular suckers
Many have
structures known as
duo-gland organs
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Consists of two different
kinds of gland cells in
one complex
Viscid gland secretes
adhesive
Releasing gland secretes
de-adhesive
Musculature
and Locomotion
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Fairly complex musculature
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Typical circular and longitudinal
Diagonal
Dorsoventral
Capable of many types of movement
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Ciliary gliding
Creeping
Swimming using dorsoventral
undulations of lateral body margins
Twisting
Somersaulting
Nervous System
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Variable
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Primitive flatworms
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Ring-like brain
One or more nerve cords
extending posteriorly
Nerve net
Dugesia, and similar worms
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Bilateral brain
Two ventrolateral longitudinal
nerve cords
Transverse commissures that
give the nervous system an
ladder-like appearance
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Sensory structures
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Usually two pigment cup ocelli
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Some possess statocysts
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Photoreceptors
Many flatworms are negatively
phototactic
Balance and orientation
Mechanoreceptors
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Senses pressure, touch, etc.
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Most posses a blind gut used for
ingestion and egestion
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Extremely long flatworms have a
through-gut
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A single layer of gastrodermis lines
gut
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Phagocytes
Glandular
Ciliated in more primitive
turbellarians
Gut design varies depending on
body size
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Microturbellaria have simple,
unbranched guts
Macroturbellaria often have
digestive ceca to increase SA for
digestion, absorption, etc.
Nutrition
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Location of mouth varies
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Usually located midventrally, but anywhere
along the ventral midline
Presence / absence / type
of pharynx varies
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Zero pharynx – no pharynx
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Simple pharynx – ciliated
tube
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Plicate pharynx – long,
muscular, protrusible tube.
Housed in a sheath when
retracted
Bulbous pharynx –
muscular sucking bulb; can
be everted in some
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Type of prey is related to pharynx structure and
body size
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Small - bacteria, unicellular algae, and protozoans
Large - small inverts such as rotifers, insect larvae,
crustaceans, and annelids
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Most are carnivorous, but some are scavengers
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Many methods of prey capture
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Wrap around prey
Entangle in mucus (sometimes toxic)
Capture with everted raptorial proboscis (has
adhesive cups or hooks)
Pin prey to substrate with adhesive organs
Some even stab with penis stylet
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May ingest whole prey items or pieces of prey
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Eversible pharynx penetrates prey or carrion, and pumps contents
into gut
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Digestion
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Extracellular initially. Pharynx is muscular and often has enzyme
releasing pharyngeal glands
Intracellular later
Can withstand long periods of starvation by reabsorbing and
metabolizing gut tissue, reproductive tissue, and various connective
tissues
Symbiosis
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Some harbor
endosymbiotic
photosynthesizers
Some are commensals of
molluscs and arthropods
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Bdelloura lives on gills of
horseshoe crabs
Shares food collected by host
Several are endoparasites
of molluscs, crustaceans,
and echinoderms
Internal Transport
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Most lack a coelom and hemal system
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Vermiform design and flatness provides large SA:vol
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Gas exchange occurs across body wall and general body
surfaces
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Nutrient circulation and diffusion
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Simple gut is not diffusion limited
Digestive ceca provide enough surface area to meet diffusion
requirements in larger worms
Gastrodermis has cilia for circulation
Excretion
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Nitrogen from protein metabolism
is in the form of ammonia
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Released along with water and
other metabolites
Protonephridia are the excretory
“organs”
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Posses lots of protonephridia, as
there is no circulatory system to
pump blood to a centralized
kidney
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Essentially a cup with ciliated
terminal cells
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Cilia draw fluid into cup
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Cup filters fluid through a meshlike structure
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Waste travels through ducts
which open to the surface via
pores
Asexual
Reproduction
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Regeneration
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Amazing regenerators (1/300th of
body can grow into an entire worm)
Active area of research
Interesting patterns of regeneration,
depending on where the animal is
cut
Clonal
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Transverse fission – posterior end
attaches, anterior end pulls and
separates
Paratomy – resembles strobilation in
cnidarians
Fragmentation – cyst forms around
fragment. Regeneration occurs and
cyst hatches
Parthenogenesis – “virgin birth”
Sexual Reproduction
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Hermaphroditic
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Male organs
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Two testes that each lead to a sperm duct…
Then to a seminal vesicle (storage)
Penis – copulatory organ that may be armed with a
sharp, hardened stylet
Female organs
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Gonopore for copulation
Copulatory bursa for short term storage of partners
sperm
Seminal receptacle for long term sperm storage
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Copulation
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Reciprocal
Rarely self fertilize
Internal fertilization
Hypodermic impregnation
Eggs
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Oviposited from gonopore
(singly, clutches, strings)
Relatively few eggs
Carefully spawned and
have protective capsules
Some FW spp. produce
both summer eggs and
resting (winter) eggs
Usually direct development
Some produce
planktotrophic larvae
Diversity of Class Turbellaria
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Order Catenulida
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Small, long, and slender
Few epidermal cilia
Head has one statocyst and
two ciliated pits
Mouth opens into simple
pharynx
Gonads unpaired
Aflagellate sperm
May reproduce via
paratomy
Catenula
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Order Acoela
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Less than 2mm long
Many cilia on
epidermis
No pharynx
Lack cellular gut
Individual germ cells
rather than gonads
Biflagellate sperm
No oviduct, eggs
rupture wall
No protonephridia
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Amphiscolops
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Order Macrostomida
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Lots of epidermal cilia
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Rhabdites
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Duo-glands
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Small paired ocelli
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Simple pharynx
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One pair of ventrolateral nerve cords w/ commissures
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Aflagellate sperm
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Macrostomum
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Order Polycladida
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Many epidermal cilia
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Rhabdites and duogland organs
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Biflagellate sperm
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Plicate pharynx
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Large, up to 30cm
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Oval, flattened
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Large gut w/ many ceca
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Some produce larvae
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May be brightly colored
(aposematic in some)
Stylochus
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Order Tricladida
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Many epidermal cilia
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Rhabdites and duo-gland organs
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Biflagellate sperm
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Plicate pharynx
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Yolky eggs
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Large gut with three branches
and many ceca
Bdelloua, Dugesia
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Order Rhabdocoela
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Several suborders
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Many epidermal cilia
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Rhabdites and duo-gland
organs
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Biflagellate sperm
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Protrusible bulbous
pharynx
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Some have proboscis which
can be everted
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Some have ventral
adhesive disc
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Mesostoma
Neodermata
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Flukes, tapeworms and
relatives belong to the taxon
Neodermata
Cellular epidermis is later
replaced with syncytium called
neodermis
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Non-ciliated
No intracellular spaces due to
syncytium
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One multinucleate cell
Nothing unwanted passes
Class Trematoda
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Flukes
Belong to Subclass
Digenea, which means
“two generations”
Endoparasites of
vertebrates
11,000 spp.
Body Form and Function
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Flat
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0.2 mm – 6.0 cm long
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Various adhesion devices
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Oral sucker surrounding
mouth
Ventral sucker
Facultative anaerobes that
mainly rely on glycolysis
Neodermis helps protect
against host’s digestive
enzymes
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Circular, longitudinal, and diagonal musculature
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Nervous system
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Similar to that of the turbellarians
Pair of anterior cerebral ganglia (brain)
Longitudinal nerve cords (variable)
Ocelli in infective larval stages (miracidia and
cercaria)
Excretory system
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Protonephridia
Two longitudinal collecting ducts
Posterior bladder
One nephridiopore
Reproduction
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Highly organized reproductive
system
Constant supply of nutrients
from host allow for large egg
production
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10,000 to 100,000 that of
turbellarians
Male system
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Two testes
Two ducts
External seminal vesicle
Cirrus sac (internal seminal
vesicle, prostate, copulatory
cirrus)
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Genital atrium shared between male and female
reproductive systems; contains one gonopore
Female system
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One ovary (germarium)
Oviduct - where seminal vesicles and vitellaria (yolk producing
structures) are added
Ootype (sac which encapsulates egg and yolk cells in protein)
Uterus
To genital atrium; out gonopore
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Some dioecious, but most are monoecious
Most cross-fertilize, but some can selffertilize
No asexual reproduction in adults
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No need, as adults usually have a constant
supply of nutrients
Asexual reproduction in some larval stages
Trematode Life Cycles
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Two or more hosts / infective larval stages
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1st intermediate host is usually a gastropod mollusc (snail)
2nd intermediate host is usually an arthropod or fish
Definitive host is a vertebrate
Aquatic cycle (may also occur on land)
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Eggs are passed in feces
If land in water, hatch as miracidia (infect snails)
Miracidium sloughs off epidermis and metamorphoses into sporocyst,
which contains many embryos
Sporocyst embryos form redia (feeding larvae with digestive system;
also produce embryos)
Redia embryos develop into cercaria (possess digestive tract, suckers
and tail)
Cercaria leave snail, swim, and infect 2nd intermediate host (usually
arthropod or fish), and encyst as metacercaria
Definitive host eats infected muscle tissue, and metacercaria develops
into adult
Free metacercaria can also be found attached to rocks or aquatic plants
Chinese Liver Fluke
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Opisthorchis (or Chlonorchis) sinensis
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Infects bile ducts of 20 million people in Asia
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Adults are 2.5 cm long
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Live up to 8 years
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Produce 4000 eggs / day
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Causes jaundice, gallstones, and possibly liver
cancer
Blood Fluke
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Schistosoma mansoni
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Found in neotropics, and Africa
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Causes schistosomiasis
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Dioecious
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300 million people worldwide
One of three worst parasitic diseases
Male and female permanently paired
Smaller female fits in groove on male
Live in intestinal veins
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Eggs laid in venules
Work way into lumen using spikes /
enzymes
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Life cycle is typical of
trematodes
Cercaria penetrate human skin
w/ enzymes and muscular
boring
Enter circulation and transform
into adults on way to intestine
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First pass through lungs and
liver
Damage to organs and vessels
from various life stages (eggs
are the worst)
Causes inflammation, necrosis,
and fibrosis
Class Monogenea
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1100 spp.
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Typically 1 – 5 mm long, but
up to 20 mm
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Mainly ectoparasites of aquatic
vertebrates; therefore rely on
aerobic metabolism
Name means “one generation,”
because they lack clonal
reproduction
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Also lack intermediate hosts
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Have a large attachment organ
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Known as a haptor
Located posteriorly
Has suckers and hooks
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Egg develops into a hooked
miracidium
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Called oncomiracidium
Has two ocelli
Adult
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Head (sometimes has muscular
oral sucker, or adhesive glands)
Trunk
Haptor
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Pharynx secretes protease which
degrades host’s epidermis
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Feed on:
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Cellular debris
Blood
mucus
Remaining body systems are
similar to turbellarians and
trematodes
Life Cycles
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Dactylogyrus
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Ectoparasite of fish gills
Serious problem in hatcheries
Leads to death by blood loss or
secondary infection
Polystoma
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Inhabits bladders of old world frogs
Synchronization of life cycles (host and
parasite eggs released at same time)
Oncomiracidia attach to tadpole’s gills
When tadpole becomes frog, parasite
migrates from gill chamber to bladder
Class Cestoda
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The tapeworms
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3400 spp.
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All endoparasites of vertebrate guts
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Lack gut entirely
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Long; can reach 25 m
Body Form
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Ribbon-like
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Three regions
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Scolex – head with hooks and suckers for
attachment
Neck – narrow growth zone containing stem
cells
Strobila – segmented trunk consisting of
proglottids (segments)
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Neodermis allows worm to avoid host’s
immune system
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Rely almost entirely on anaerobic
respiration
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Muscular, nervous, and protonephridial
system is similar to that of turbellarians
and trematodes
Reproduction
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Strobilation occurs at neck region
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Each proglottid has a miniature reproductive system
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When one worm is present
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Cross fertilize
Worms have proglottids that are in different stages of sexual
maturity
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Self fertilize using one proglottid
Serf fertilize between two proglottids
When two or more worms are present
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During copulation, cirrus is everted and inserted into gonopore
Eggs constantly being produced
Mature (gravid) proglottids break off from strobila and are passed in
feces
Life Cycles
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Usually two or more hosts
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Definitive host always a vertebrate
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Aquatic and terrestrial life cycles
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Diphyllobothrium is a fish tapeworm that can infect humans
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Eggs in feces are deposited in water
Hatch into a ciliated, swimming oncosphere larva (has hooks)
Oncosphere is ingested by a copepod
Oncosphere moves to hemocoel of copepod, sheds ciliated epidermis,
replaces with neodermis, and develops into a procercoid larva
Fish eats copepod
Procercoid migrates from gut of fish to muscle, and transforms into a
juvenile called a plerocercoid (metacestode)
Development is completed in the gut lumen of a fish-eating vertebrate
after ingestion of infected muscle tissue
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Taenia is a terrestrial
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Can reach 20 m but is typically
3 to 5 m
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Lifecycle
tapeworm that infects
domestic animals and humans
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Eggs are removed in feces
Animal ingests eggs and
oncosphere bores through
intestinal wall and migrates to
skeletal muscle
Oncosphere develops into a
cysticercus (another
metacestode w/ inverted
scolex)
Development is completed in
the gut lumen of a vertebrate
after ingestion of infected
muscle tissue
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Tapeworm infections cause diarrhea,
weight loss, and lethargy
Occasionally humans accidentally
serve as an intermediate host, by
becoming infected with a cysticercus
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Echinococcus is often responsible for this
Cysticercus ends up in lung, liver, heart,
brain, etc.
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Swells up
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Known as a hydatid or hydatid cyst
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Must be removed surgically
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Some mortality