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Z202
Chapter 14
Acoelomate Animals
I.
General Features: Acoelomate Bilateria
A. Position in the Animal Kingdom
1. Acoelomates –
a. Solid body w/out
Coelom
2. Simplest animals w/ Bilateral Symmetry
a. most efficient form for active directed movement
- anterior-posterior orientation
- dorsal and ventral sides
b. derived from Cephalization
(concentration sense organs on head)
(provides anterior/posterior ends)
3. Other features:
a. Protostomes w/ spiral cleavage
b. Triploblastic (all 3 germ layers)
c. organ-system level of organization
d. more complex of nervous system
e. Simplest excretory/osmoregulatory, circulatory and
digestive system (complete)
II.
Protostomia Clades
A. Protostomis divided into 2 large clades:
1. Lophotrochozoa – share either:
♦ horse-shoe shaped feeding structure
(lophophore)
♦ particular larval form called trochophore
2. Ecdysozoa – possess a cuticle that is molted as
they grow
III.
Lophotrochozoa - Phylum Platyhelminthes
A. Characteristics
- range mm to many meters in length (avg 1-3 cm)
- free-living (turbellarians/planarians, some larvae stages)
OR
- parasitic (flukes & tapeworms)
B. Form & Function
1. Tegument & Muscles
- Turbellarians ciliated epidermis w/
protective mucous sheath & adhesive organs
- Parenchyma cells fills spaces of body @ muscle fibers acoelomates
- Some have Synctial Epidermis (appear multinucleated)
2. Nutrition & Digestion
- range from no digestive to simple extracellular incomplete
(mouth/pharynx/gastrovascular cavity divided into intestine)
- planarians have protruding pharynx
& 3 branched intestines
(1 anterior, 2 posterior)
3. Excretion & Osmoregulation
- most have protonephridia w/ flame cells
(flagella drives fluid down collecting ducts - walls reabsorb)
(collecting ducts join & empty at nephridiopores)
- metabolic wastes removed by diffusion through body walls
(primarily ammonia)
- elaborate ducting since no circulatory system
4. Nervous System
- subepidermal nerve plexus resembles nerve net/ladder
- 1 to 5 pairs of longitudinal nerve cords lie under muscle
layer
- Brain is bi-lobed ganglion anterior to ventral cords
- Nerves (unidirectional) specialized: sensory, motor,
association
5. Sense Organs
- active locomotion favored
cephalization & sense organ
evolution
- sense organs range:
light-sensitive eyespots (ocelli)
touch & chemical receptors cells abundant
statocyts for equilibrium
rheoreceptors for sensing H2O current
nerve endings in oral suckers, holdfast organs &
genital pores in parasites
6. Reproduction & Regeneration
- Fission (regenerate 1/2 of each side)
- Some asexual reproduction (may occur in host)
- Nearly all monoecious but
cross-fertilize
- Sexual reproduction
(develop F and M organs)
- tapeworms attached multiple reproductive segments
(proglottid)
- life cycle often requires
multiple hosts
♦ liver fluke – requires
human & snail hosts
7. Classification
Class Turbellaria
(planaria)
Class Trematoda
(parasitic flukes)
Class Cestoda
(tapeworms)
♦ life cycle
includes multiple
hosts:
1. humans
2. cattle/sheep/
pork
IV.
Phylum Mesozoa
A. “Middle Animals”
- termed coined in 1876 as “missing link” between protozoa &
metazoan (true animals)
- minute, ciliated “worms” with extremely simple level of
organization
- all live as parasites or symbionts in marine invertebrates (0.5
to 7 mm in length for most)
- composed of 20-30 cells arranges in 2 layers (not similar to
germ layers)
- 2 classes: Rhombozoa and Orthonectida
V.
Phylum Nemertea
"Ribbon Worms"
A. General Characteristics
- over 1000 species
- most are less than 20cm long (some stretch to 60m) & all
have ciliated epidermis (also glandular)
- most marine; some found in moist soil & freshwater
- body plan similar to planaria
Exceptions:
♦ digestive system is extracellular complete
♦ reproductive – dioecious (sexual reproduction prominent)
- most obvious feature is proboscis (extend to capture prey)
- simplest animals w/ blood-vascular system (circulatory)
- most are active predators; some scavengers or parasites
VI.
Phylogeny & Classification
A. Phylogeny
- body shape & metabolic requirements ideal for
parasitic lifestyle
- Ribbon worms evolve beyond flatworms for a narrow
predatory niche
- rDNA, embryonic development, & mesodermal origin
indicate acoelomates are not monophyletic
B. Classification
Phylum Platyhelminthes
Class Turbellaria
(planaria)
Class Trematoda
(parasitic flukes)
Class Cestoda
(tapeworms)
Phylum Mesozoa
Phylum Nemertea
(ribbon worms)