The First Hunters: Flatworms! Phylum Platyhelminthes
From sponges to cnidarians, these animals randomly come about their food and their prey.
Big Question: From an evolutionary perspective, what adaptations are needed to become hunters?
We are starting to see these adaptations in flatworms. Flatworms are:
Unsegmented:
Bilaterally Symmetrical:
Cephalization:
Have 3 Germ Layers:
1. Ectoderm- skin and muscles
2. Mesoderm- excretion and nerves
3. Endoderm- digestion
Why are the 3 Germ layers an important evolutionary concept?
Diagrams:
Flatworm/ Flame Cell
So, in short, the Phylum Playthelminthes is the first group of animals that have a semblage of internal
organs.
How do flatworms acquire energy?
1. Free-living flatworms:
2. Parasitic flatworms:
How would a branched gastrovascular cavity be beneficial to a small animal like a flatworm?
How do flatworms reproduce?
Most free living flatworms are hermaphroditic. Worms pair up and exchange sperm (re: Penis
Fencing).
Flatworms also reproduce ASEXUALLY by fission. This is similar to bacterial fission as worms splits in
two and each half grows into new individuals. Others, like a tape worm, simply breaks into pieces and
each piece grows into a new individual.
How do flatworms expel wastes?- flatworms have specialized cells called FLAME CELLS.
Flame Cells: cells that have a specialized function that remove metabolic wastes like ammonia and
urea. These cells form a network of tubes that empty wastes through the ectoderm of the flatworm.
How do flatworms respond to the environment?
-Group of nerve cells concentrated in the anterior end of the body
-Eyespots (group of light sensitive cells) located near head.
-Chemo-sensitive cells located along the length of the body.
Diagram:
LIFE LOG: BIG UNDERSTANDING: Design an experiment that would investigate how sensitve the
flatworm’s eyespots are. Your group must have a hypothesis, an independent variable, a dependent
variable, a control and a plan to collect data.
Flatworm Diversity:
Over 20 000 species belong to Phylum Platyhelminthes. These species are divided within 3 classes:
Class Turbellarians: Free living fatworms. Can live in Fresh or Marine water.
Class Trematoda or Flukes: Parasitic flatworms hat infect the internal organs of their host.
Class Cestoda or Tapeworms: Long flat parasitic worms that are adapted to life inside the intestines of
their host.
Phylum Nematoda: Roundworms; aka Nematodes
Like bacteria, roundworms can be found in surprising number in a small area. A cubic meter of soil
can be home to about a million roundworms!
How can they reach such a great number? A roundworm can lay up to 200 000 eggs in a single day!
Classification: Roundworms are:
I. Unsegmented worms with tapered ends.
II. Three Germ Layers:
1. Ectoderm
2. Mesoderm
3. Endoderm
III. A Pseudocoelom: a space that develops between the mesoderm and the endoderm.
Evolutionary speaking, this space is extremely important.
Quick Question: What do you think a pseudocoelom can lead to?
IV. A digestive tract with two openings. Notice every animal we’ve uncovered thus far
(sponges, cnidarians and platyhelminthes) poop with the same opening they eat with. Luckily, this
body plan has withstood the test of natural selection and our body plan has evolved from it.
Diagram:
How do nematodes acquire energy?
-Free living nematodes are carnivores that grasp their prey with their mouths. Their prey can be small
animals, algae, fungi or bacteria.
-Some nematodes are parasitic and feed off host cells.
How do nematodes excrete wastes and respire?
- Roundworms use their body walls to get rid of wastes and to respire. They depend on the process of
diffusion.
Quick Question: Define Diffusion:
How do nematodes respond to their environment?
-Ganglion run up and down the roundworms body (similar to that of the flatworm). They also have
chemoreceptors along the side of their body to detect prey.
How do nematodes reproduce?
-Nematodes reproduce sexually. Most species have separate males and females. Parasitic
roundworms often have complex life cycles that require multiple hosts.
Assignment: You are a Marketing employee for Health Canada. You are asked to Create a Wanted
Poster for two Parasitic Flatworms and one Parasitic Roundworm to educate the public on how to
avoid these parasites. A representative of Class Trematoda, Cestoda, and Phylum Nematoda must be
presented. For the poster, the parasitic cycle should be diagramed (depicting their crime and their
getaway against the host species) and explained as well as information of the disease or symptoms
they cause. You should also provide ways for the public to protect themselves from the potentially
dangerous parasites. Chapter 27 in your textbook can be your first source.
Here’s how you’ll be assessed:
Excellent 18-20 (A)
 Wanted Poster is clear,
visually engaging and
aesthetically pleasing.
 Evidence of time spent.
 Has 6+ pictures for each
worm on the wanted poster.
 Pictures are large, clear and
well labeled.
 Shows a deep understanding
of the parasitic cycle.
 6+ sources in bibliography.
 Written explanation is well
written, concise with little or
no grammatical or spelling
errors.
 Written explanation is 300+
words.
 Has the ability to verbally
explain the parasitic cycle and
possible ways of defending or
ridding of the parasite.
Self Assessment: -_______/20:
Explain:
Mr. Jay’s Assessment:
Comments:
B
I
N
B
E
T
W
E
E
N
Acceptable 11-14 (C)
 Wanted poster is messy and at
times unclear.
 Evidence of poster being put
together last minute.
 3- pictures for each flatworm
on the wanted poster.
 Pictures are not very clear or
not well labeled.
 Shows a satisfactory
understanding of the parasitic
cycle.
 3- sources in bibliography.
 Written explanation is at
times unclear and convoluted,
with grammatical and spelling
errors.
 Written explanation 180words.
D
Poor 0-7 (F)
 Poster is incomplete or
incomprehensible.
 Has 1- pictures that are
I
unlabelled or unclear.
N
 Shows little understanding of
the parasites.
B
 Shows little research into a
E
specific worm.
T
 Written explanation is poorly
W
done with many grammatical
E
and spelling errors.
E

Written explanation is 100N
words.
 Unable to explain their food
chain.