acoelomate bilateral animals

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
Radially symmetrical 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
These acoelomate 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
Many can 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
All monogeneans
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
Ribbon worms,
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.