Phylum

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Phylum Platyhelminthes Flatworms
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I. Phylum Platyhelminthes (platy flat
helminthes worm)
A. Characteristics

• dorsoventrally flattened
• unsegmented
• triploblastic three tissue layers it is the
  first time that we see mesoderm in animals
• show cephalization head region
• have a primitive gut
• have some organs present protonephridia and
  flame cells for excretion and osmoregulation
• no circulatory or respiratory systems
• most are monoecious

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9. only one opening to digestive system so are
said to have two-way digestion 10. no body cavity
tissue layers are stacked one on top of the
other they have no coelom
a. these type of animals are said to be
acoelomates
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11. many species are parasitic 12. bilateral
symmetric (have bilateral symmetry) 13. do have
a nerve net with collections of nerve fibers in
the head called ganglia 14. some free living
species have eyespots that can detect the
difference between light and dark. These eyespots
are called ocelli
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B. Class Turbellaria (turbulence set off
by whipping cilia)

• free-living worms (non-parasitic)
• there are both fresh and marine species
• total nearly 300 species

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4. exist as predators and scavengers, but not
parasites 5. have ciliated epidermal cells which
allow these organisms to move 6. rhabdites are
special cells on the ventral side of the worm
that can swell and form a mucous protective slime
over the worm 7. parenchyma (literally means
filling) this is the space filling tissue of
mesoderm origin forming the middle layers of the
worm 8. digestive system ranges from simple in
some species to complex in others 9. include the
freshwater Planaria and the many beautiful marine
species of flatworms that exist found in the
oceans
10. also includes the terrestrial land planarian
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11. the small structures off the head are
called auricles and have sensory organs located
in them
12. feed through a tube called the pharynx
Auricles
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13. Reproduction

• asexually by simply binary fission (splitting)
  into two zooids
• have great powers of regeneration

c. sexually (they are monoecious), but cannot
self-fertilize

• Produce reciprocal sperm
• copulatory organ is the penis
• eggs are contained in a structure called a cocoon
  
• larva hatch as free-living forms called Mullers
  larva

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Mating Planaria
Mullers Larva
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C. Class Monogenea
1. includes the monogenetic flukes that have but
one generation in their life cycle
D. Class Trematoda
1. class of the greatest number of flukes with
8,000 species
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2. they attach by their mouths and also a
structure at their posterior end called an
acetabulum
(see diagram in zoology coloring book )
3. intermediate host is a snail in almost every
case many also have a fish as an intermediate
host 4. these flukes live in the bile ducts and
liver of their host 5. Some species of flukes
are responsible for frog abnormalities in
northern ponds of the U.S.
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E. Class Cestoidea

• Cestodes are the tapeworms
• they are the most highly specialized of the
  Platyhelminthes
• there are 3500 species and all are endoparasites
  (living inside their hosts)
• they have lost their digestive systems and are
  thus said to be degenerative parasites
• since they cannot live outside their host, they
  are said to be obligate parasites

6. they lack a mouth, anus, or any digestive
structures
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7. cestodes are monoecious 8. the head of a
tapeworm is called a scolex
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9. each individual body segment is called a
proglottid

• each proglottid is a reproductive machine having
  both male and female reproductive organs in the
  same structure
• younger, smaller proglottids form near the scolex

c. older proglottids, further from the scolex,
often are gravid (full of eggs) d. the strobila
is the main part of the body consisting of many
proglottids
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10. Taenia solium is the pork tapeworm

• can be up to 10 to 12 meters long
• if the larva burrow into the muscle tissue they
  are called a cysticercus
• if the cysticercus burrows into the brain, it can
  be fatal

11. life cycle of the beef tapeworm
Taeniarhynchus saginatus
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adult tapeworm (3 meters in small
intestine)
eggs released in feces
infected meat ingested
Onchosphere larva


cyticercus (bladder worm burrowed in muscle)
Cattle graze on contaminated grass
Larva cycles through the intestine, then into the
blood, then burrows into the muscle tissue
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IV. Phylum Nemertea

• known as the ribbon worms or also the proboscis
  worms
• Characteristics

• Triploblastic
• acoelomates
• bilateral symmetric
• complete digestive tract with both mouth and
  anusone-way digestion
• have protonephridia for excretion
• have a closed circulatory system

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7. have a food gathering device called a
proboscis than can be extruded and act somewhat
like an elephants trunk to capture food
a. the proboscis is protected within a sheath
called a rhynchocoel when not extruded
proboscis