coeloms

1. Flatworms, Mesozoans, and
Ribbon Worms
Chapter 14

2. Bilateria
 Most animals have
bilateral symmetry.
 The vast majority of
animal species
belong to the clade
Bilateria, which
consists of animals
with bilateral
symmetry and
triploblastic
development.

3. Bilateral Symmetry
 Radiallysymmetrical animals have the
world coming at them from all directions.
 They can catch prey coming from any
direction.
 Animals that begin to move about
actively seeking food need a different
body organization.
 Distinct
head end with sensory structures.
 Cephalization

4. Bilateral Symmetry
 Animals with bilateral symmetry have a
distinct head end and can be divided into right
and left halves.

5. Acoelomate Bilateral Animals
 Animals that have no space between
their gut and body wall are said to be
acoelomate.
 These animals are also triploblastic –
they have three embryonic germ layers.
 Organ-system level of organization –
more division of labor among their
organs.

6. Acoelomates
 Although
flatworms undergo triploblastic
development, they are acoelomates.

7. Acoelomates
 Theseacoelomate phyla are
protostomes and have spiral cleavage.
 Most have determinate cleavage.
 These are the simplest animals with an
excretory system.
 Acoelomate phyla belong to the
superphylum Lophotrochozoa

8. Phylum Acoelomorpha
 Group contains ~350 species.
 Members were formerly in Class
Turbellaria within phylum
Platyhelminthes Small flat worms less
than 5 mm in length.
 Typically live in marine sediments; few
are pelagic.
 Some species live in brackish
water.
 Most symbiotic but some parasitic.
 Have a cellular ciliated epidermis.
 Parenchyma layer contains small
amount of ECM and circular,
longitudinal, and diagonal muscles.

9. Phylum Acoelomorpha - Digestion
and Nutrition
 Incomplete digestive
system - no anus.
 In many acoels, the
gut and pharynx are
absent.
 Phagocytotic cells
digest food
intracellularly when
food is passed into
temporary spaces.

10. Phylum Acoelomorpha -
Reproduction
 Monoecious
 Female produces yolk-filled, endolecithal
eggs.
 Following fertilization some or all
cleavage events produce a duet-spiral
pattern of new cells.
 May be a defining character for
acoelomorphs.

11. Phylum Platyhelminthes
 Members of
phylum
Platyhelminthes
live in marine,
freshwater, and
damp terrestrial
habitats.

12. Phylum Platyhelminthes
 Flatworms are
flattened
dorsoventrally and
have a
gastrovascular
cavity.
 Extracellular
digestion.
 Undigested food is
egested through the
pharynx.

13. Phylum Platyhelminthes
 The osmoregulatory
system consists of
protonephridia
(excretory or
osmoregulatory
organs closed at the
inner end) with
flame cells.
 Most metabolic
wastes removed
by diffusion across
the body wall.

14. Phylum Platyhelminthes
 The nervous
system
consists of a
ladder-like
network of
nerves and a
bilobed brain.
 Many have
large ocelli –
light sensing
organs.

15. Phylum Platyhelminthes
 Manycan reproduce
asexually as well as
sexually.
 Asexual
reproduction via
fission.
 Sometimes the new
individuals remain
attached – chains
of zooids.
 Monoecious

16. Taxonomy
 Flatworms (phylum Platyhelminthes) are
divided into four classes:
 Class Turbellaria – ex. Planaria
 Not monophyletic
 Class Trematoda – parasitic flukes
 Class Monogenea – parasitic monogenetic
flukes
 Class Cestoda - tapeworms

17. Phylum Platyhelminthes

18. Class Turbellaria
 Turbellarians are nearly all free-living and
mostly marine.

19. Class Turbellaria
 The best-known turbellarians, commonly
called planarians, have light-sensitive
eyespots and centralized nerve nets.

20. Class Trematoda
 Trematodes live as parasites in or on
other animals.
 They parasitize a wide range of hosts.

21. Class Trematoda
 Subclass Digenea,
digenetic flukes,
have a complex
life cycle with a
mollusc (snail) as
the first host and a
vertebrate as the
final, or definitive,
host.

22. Class Monogenea
 Allmonogeneans
are parasites.
 Often found in the
gills or external
surfaces of fishes.

23. Class Cestoda
 Tapeworms (Class
Cestoda) are also
parasitic and lack a
digestive system.
 The scolex is equipped
with suckers and hooks
for attachment to the
host.
 Each proglottid
contains a set of
reproductive organs.

24. Class Cestoda
 Cestodes usually
require at least two
hosts.
 Adult cestodes are
parasites in the
digestive tracts of
vertebrates.

25. Phylum Mesozoa
 Phylum Mesozoa is considered a “missing link”
between protozoa and metazoa.
 Have a simple level of organization.
 Minute, ciliated, and wormlike animals.
 All live as parasites in marine invertebrates.
 Most composed of only 20 to 30 cells arranged in two
layers.
 Layers are not homologous to germ layers of other
metazoans.
 Two classes, Rhombozoa and Orthonectida, are so
different that some authorities place them in separate
phyla.

26. Phylum Mesozoa
 Rhombozoans live in kidneys
of benthic cephalopods.
 Adults called vermiforms and
are long and slender.
 Inner, reproductive cells give
rise to vermiform larvae.
 When overpopulated,
reproductive cells develop into
gonad-like structures
producing male and female
gametes.
 Larvae are shed with host
urine into the seawater.

27. Phylum Mesozoa
 Orthonectids parasitize a variety of
invertebrates.
 Reproduce sexually and asexually.
 Asexual reproduction consists of a
multinucleated mass called a plasmodium.

28. Phylogeny of Mesozoans
 Some consider these organisms primitive flatworms
and place them in phylum Platyhelminthes.
 Molecular evidence groups them with flatworms in
superphylum Lophotrochozoa.
 However, molecular phylogeny that included an
orthonectid and two species from a rhombozoan
subgroup, the dicyemids, did not show members of the
two classes to be sister taxa.
 The phylum may not be monophyletic.

29. Phylum Nemertea
 Ribbonworms,
phylum Nemertea,
use a proboscis to
capture prey.
 Almost completely
marine.
 Active predators.
 General body plan
similar to
turbellarians.

30. Phylum Nemertea
 An anus is present providing these worms
with a complete digestive system.
 Nermeteans are the simplest animals to have
a closed loop blood-vascular system.

31. Phylogeny
A planuloid ancestor (like the planula
larva of cnidarians?) may have given
rise to a branch of descendents that
were sessile or free floating and radial
Cnidaria.
 Another branch acquired a creeping
habit and bilateral symmetry
Bilateria.