Acoelomate Bilateral
Animals
Acoelomate Bilateral Animals
• Consist of phyla:
– Phylum Platyhelminthes
– Phylum Nemertea
– And others
Acoelomate Bilateral Animals
1. Simplest organisms to
have bilateral
symmetry
2. Triploblastic
3. Lack a coelom
4. Organ-system level of
organization
5. Cephalization
6. Elongated, without
appendages
Reproductive and
osmoregulatory systems
Bilateral Symmetry
• Divided along sagittal plane into two mirror
images
– sagittal= divides bilateral organisms into right
and left halves
• Anterior= head end
• Posterior= tail end
• Dorsal= back side
• Ventral= belly side
Review
Bilateral animals
• Bilateral symmetry = important evolutionary
advancement
– Important for active, directed movement
• Anterior, posterior ends
– One side of body kept up (dorsal) vs. down
(ventral)
Directed movement evolved with anterior
sense organs cephalization
Cephalization
– specialization of sense organs in head end of
animals
• Acoelomates lack a true body cavity
– Solid body
Acoelomate
Phylum Platyhelminths
Or not shown here
Nemerterean
– no cavity b/w the digestive tract and outer
body wall
This is a round worm
Different Phylum
Acoelomates are triploblastic
• Triploblastic (3 germ layers)
– Germ layer= layers in embryo
that form the various tissues
and organs of an animal body
3 germ layers
• Ectoderm
– Outermost germ layer
– Gives rise to outer covering of animal ie. epidermis
• Endoderm
– Innermost germ layer
– Gives rise to inner lining of gut tract
• Mesoderm
– Middle germ layer
– b/w ectoderm and endoderm
– Gives rise to various tissues/organs (ie. muscles)
Acoelomate animals have an
organ-system level of
organization
Acoelomate animals have an organsystem level of organization
• Organ-system
– Different organs
operate together (ie.
excretory system,
nervous system)
– mesodermal tissue
gives rise to
parenchyma
Digestive tract and
nervous system
Polyclad
• From Red Sea
http://www.rzuser.uni-heidelberg.de/~bu6/
Phylum Platyhelminthes
Flatworms
Free living
Parasitic
• From Atlantic ocean
http://www.rzuser.uni-heidelberg.de/~bu6/
Phylum Platyhelminthes
• Flattened
dorsoventrally
– flatworms
• 34,000 species
• Gastrovascular cavity
(if present) has only
one opening (mouth =
anus)
• Mostly monoecious
Phylum Platyhelminthes
• First phylum that has an Organ
systems present
– derived mesodermally (parenchyma):
• Muscular system
• Digestive system (incomplete; gastrovascular
type) (absent in some)
• Nervous system
• Excretory system (absent in some)
• Reproductive system
Phylum Platyhelminthes
• Organ systems absent:
– Circulatory
– Respiratory
Rely on diffusion
Phylum Platyhelminthes (cont’d)
• Divided into 4
classes:
– Class Turbellaria
(mostly free-living
flatworms)
– Class Trematoda
(parasitic flukes)
– Class Monogenea
(ectoparasitic
flukes)
– Class Cestoda
(tapeworms)
Hymenolepsis- rat tapeworm
Class Turbellaria
• Mostly free-living
flatworms
• Marine (mostly) or
freshwater bottomdwellers
• Predators and
scavengers
• First group of
bilateral
symmetrical animals
Planarian genus Dugesia
re 33.10
• The best-known turbellarians, commonly called
planarians
– Have light-sensitive eyespots and centralized nerve
nets
Digestion is completed within
the cells lining the gastrovascular cavity, which has
three branches, each with
fine subbranches that provide an extensive surface area.
Pharynx. The mouth is at the
tip of a muscular pharynx that
extends from the animal’s
ventral side. Digestive juices
are spilled onto prey, and the
pharynx sucks small pieces of
food into the gastrovascular
cavity, where digestion continues.
Undigested wastes
are egested
through the mouth.
Gastrovascular
cavity
Eyespots
Ganglia. Located at the anterior end
of the worm, near the main sources
of sensory input, is a pair of ganglia,
dense clusters of nerve cells.
Ventral nerve cords. From
the ganglia, a pair of
ventral nerve cords runs
the length of the body.
Class Turbellaria (cont’d)
• Move by muscles, ciliated epidermis
/gastrovascular cavity
Class Turbellaria (cont’d)
• Freshwater
turbellarians adapted
osmoregulatory
structures
– Protonephridia
• protos= first
• nephros= kidney
• network of fine tubules
running down sides of
organism
Class Turbellaria (cont’d)
– Flame cells= branch
from tubules
• Ciliary projections
drive fluid down
tubule
– Tubules open to
outside=
nephridiopore
Class Turbellaria (cont’d)
• nervous system with nerve ganglion
– ganglion- aggregation of nervous tissue
• Cephalization- cerebral ganglion= primitive brain
Class Turbellaria (cont’d)
• Ocelli= light-sensitive eyespots
Turbellarian Reproduction
• Asexual (fission)
– transverse
• Sexual
– Monoecious
(mostly)
– Cross-fertilization
Reproductive and
osmoregulatory
systems
Other 3 classes:
– Class Trematoda
– Class Monogenea
– Class Cestoda
• All parasitic
• lack cilia
• Have unusual body
covering: tegument
• Outer zone of tegument
(glycocalyx)
• consists of proteins
and carbohydrates
• aids in transport of
nutrients, waste, gases
• Protection against host
defenses
Class Trematoda
• Parasitic flukes
• Endoparasites
– Hooks, suckers,
increased
reproductive capacity
•
1mm-6cm long
•
Complex life
cycle:
1. Definitive host
(primary/final
host)
–
–
where parasite
matures and
reproduces
(sexually) (eggs
released)
vertebrate
2. Intermediate
host
–
Mollusc (ie. snail)
–
Hosts in which
larval stages
develop and
undergo asexual
reproduction
–
Results in an
increase in the
number of the
individuals
• Trematodes that parasitize humans
– Spend part of their lives in snail hosts
1 Mature flukes live in the blood vessels of the human
intestine. A female fluke fits into a groove running
the length of the larger male’s body, as shown in
the light micrograph at right.
Male
Female
5
1 mm
These larvae penetrate
the skin and blood
vessels of humans
working in irrigated
fields contaminated
with infected human
feces.
2 Blood flukes reproduce
sexually in the human host.
The fertilized eggs exit the
host in feces.
3 The eggs develop in
water into ciliated
larvae. These larvae
infect snails, the
intermediate hosts.
Figure 33.11
4 Asexual reproduction
within a snail results in
another type of motile
larva, which escapes from
the snail host.
Snail host
Chinese Liver Fluke
• Infects 30 million
people in eastern
Asia
• Lives in ducts of
liver
– Eats epithelial
tissue, blood
• Definitive host:
– Humans, dogs,
cats
• 2 intermediate
hosts:
– snail
– fish
Class Monogenea
• Parasitic flukes
• Mostly ectoparasites
• Single host, mostly
fish
Class Cestoda
• Tapeworms
• Endoparasites
• Vertebrate host
– Live in digestive
tract
• 1 mm- 25m long
(EWWWW!!)
Hymenolepsis- rat tapeworm
Tapeworm
• Tapeworms
– Are also parasitic and lack a digestive
system
Proglottids with
reproductive structures
200 µm
Scolex
Figure 33.12
Hooks
Sucker
Class Cestoda
• Highly
specialized
• Lack mouth,
digestive tract
– Absorb nutrients
across body wall
• Hooks and
suckers
– “head”= scolex
Hymenolepsis- rat tapeworm
• Adult tapeworms consist of long series of
repeating units= proglottids
• Chain of proglottids= strobila
• Tapeworms are monoecious (mostly)
– Mostly cross-fertilization
• No specialized sense organs
scolex
• Cestodes depend
on host digestion
– Small molecules
in host intestine,
liver
Beef Tapeworm
• Definitive
host= human
• Intermediate
host= cattle
Phylum Nemertea- ribbon
worm
• Triplobastic, acoelamate
•
bilateral symmetry
• Unsegmented
• Ciliated epidermis
• Closed circulatory
• usually <20cm
• Marine mud, sand
• Elongate, flattened
worms
Phylum Nemertea (cont’d)
• Unlike the platyhelminthes, Complete
digestive tract, with anus
– One-way
– More efficient; allows larger growth
Nemertea
Phylum Nemertea (cont’d)
• Cerebral ganglion, longitudinal
nerve cords
• Long proboscis used in
carnivorous species
• Two lateral blood vessels yet
no heart
• Dioecious
– “two” “house”
– Male and female organs in
separate individuals
Sea serpents?