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Flatworms: Read passage and answer questions
Name___________________________
The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of
relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the
largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp
terrestrial environments. Most are free-living forms, but many are parasitic on other animals.
There are four classes: Trematoda (Flukes), Cestoda (Tapeworms), Monogenea, and Turbellaria.
Description
The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to
bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs.
This means their organ systems form out of three germ layers: an outer ectoderm and an inner
endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis,
while cestodes and trematodes are covered with a cuticle.
Body functions
There is also no true body cavity (coelom) except the gut and hence they are acoelomates. The
interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called
parenchyma tissue. There is no true circulatory or respiratory system, but like all other animals,
flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells
to help distribute nutrients, gases, and waste products.
Flatworms respire at their integument; gasses diffuse directly across their moist outer surface.
This type of system is called integumentary exchange.
However, flatworms do have a bilateral nervous system; they are the simplest animals to have
one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of
some species even has a collection of ganglia acting as a rudimentary brain to integrate signals
from sensory organs such as eyespots.
Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate
undigested particles of food simultaneously, as most animals with tubular guts can. This blindended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few
particularly long flatworms or those with highly branched guts, there may be one or more anuses.
A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be
unrelated to the other Platyhelminthes (see below).
Despite the simplicity of the digestive chamber, they are significantly more complex than
cnidarians in that they possess numerous organs, and are therefore said to show an organ level or
organization. Mesoderm allows for the development of these organs, and true muscle. Major
sense organs are concentrated in the front end of the animals for species who possess these
organs.
Muscular contraction in the upper end of the gut causes a strong sucking force allowing
flatworms to ingest their food and tear it into small bits. The gut is branched and extends
throughout the body, functioning in both digestion and transport of food. Flatworms exhibit an
undulating form of locomotion.
Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm.
When two flatworms mate, they exchange sperm so both become fertilized. They usually do not
fertilize their own eggs. Turbellarians classified as planarians (usually freshwater, non-parasitic)
can also reproduce asexually by transverse fission. The body constricts at the midsection, and the
posterior end grips a substrate. After a few hours of tugging, the body rips apart at the
constriction. Each half grows replacements of the missing pieces to form two whole flatworms.
This also means that if one of these flatworms is cut in half, each half will regenerate into two
separate fully-functioning flatworms.
Depending on species and age, individuals can range in size from almost microscopic to over 20
m long (some tapeworms can attain this length).
Questions:
1. Are flatworms vertebrates or invertebrates? ______________________
2. Flatworms can be free-living, but many are ______________ on other animals.
3. The four classes of Platyhelminthes are: _______________, __________________,
_________________, and ______________________.
4. What kind of symmetry do flatworms have? ____________________
5. Flatworms have 3 germ layers, which makes them ______________, and these germ
layers are: _________________, ________________, and _______________.
6. Turbellarians generally have a _____________ epidermis, while cestodes and trematodes
are covered with a _______________.
7. Flatworms have no true body cavity and therefore are _______________.
8. Respiration occurs at the ___________ where gasses diffuse across their moist outer
surface.
9. Flatworms are the simplest animals to have a _____________ which branches repeatedly
in an array resembling a ladder.
10. The collection of __________ act as a brain to integrate signals from sensory organs.
11. One example of sensory organs would be the ocelli, also known as ____________.
12. The digestive tract has _______ opening(s).
13. Flatworms show _________ level of organization.
14. Organs and muscles develop from which germ layer? _________________.
15. Flatworm reproduction is _____________, meaning each individual produces both eggs
and sperm.
16. Planarians (class Turbellaria) can reproduce asexually by __________________.