Acoelomates
Phylum Platyhelminthes
Phylum Nemertea
Phylum Gnathostomulida
Taxonomy
Grade Bilateria
Division Protostomia
Type title here
Acoelomates
Example:
Platyhelminthes
Pseudocoelomates
Example:
Nematoda
Division Deuterosomia
Eucoelomates
Examples:
Mollusca, Arthropods, Annelida
Examples:
Echinodermata, chordata
Acoelomate Phyla
• Platyhelminthes
– Flat worms
• Nemertea
– Ribbon worms
• Gnathostomulida
– Jaw worms
Why bilateral symmetry?
Movement toward prey or host
– Cephalization
– Directional sense organs
• Chemoreceptors
• Ocelli (light sensing eyespots)
• Rheoreceptors (sense water currents)
Acoelomates Key Features
• Three germ layers
– Ectoderm, endoderm, mesoderm
– Mesoderm forms muscle and mesenchyme
• Organ-system level of organization
– Cephalization
– Excretory system
– Some have circulatory and one-way alimentary
canal
Acoelomates Body Plan
Phylum Platyhelminthes
• Specialized Cells
–
–
–
–
Rhabdites - secrete mucous sheath
Tegument - outer covering of syncytial cells
Flame cells – Osmoregulation
Some have endolecithal egg cells – yolk
contained within egg cell
Platyhelminthes Cross Section
Tegument
• Syncytium
– Apical cell membrane
missing
Flame Cell
• Osmoregulation
– Beating flagella creates negative pressure to
draw fluid into chamber
– Water is directed through channels then through
an external pore
Major Classes of Platyhelminthes
• Class Turbellaria
– Planaria – free living
• Class Trematoda
– Liver flukes – endoparasites
• Class Monogenea
– Fish ectoparasites
• Class Cestoda
– Tape worms – endoparasites
Class Turbellaria
• Key Features
–
–
–
–
–
Ladder like nervous system
Eye spots (Ocelli)
Auricles - chemoreceptors
Free living flatworms
Muscular and ciliary
movement
– Carnivorous
Planarian Body Plan
Class Trematoda
• Similar in structure to Turbellaria
• Special adaptation for endoparasitic life style
– Cyst glands
– Suckers and hooks for griping host
• Digenetic – complex life cycle with multiple
hosts
Liver Fluke
Liver Damage Caused By Flukes
• Scar tissue
• Blocked bile ducts
Flukes in Liver Section
Fasciolopsis - Anterior End
Schistosoma mansoni
• Blood fluke
• Male and female are dioecious
Schistosoma cercaria
Class Cestoda
• Tapeworms - endoparasites
• Key Features
– Lack a digestive system
– Composed of reproductive units called proglottids
– Adult tegument covered in microvilli-like
projections – increases surface area, non-ciliated
Tapeworm
• New proglottids are
added just behind the
scolex
Tapeworm Tegument
• Microvilli help with food
adsorption
• Sensory cells – no other
sense organs
Mature Proglottid
• Reproductive unit
• Mature proglottids
are either shed or
produce shelled
embryos
• Each proglottid has
both male and
female organs
Tape Worm
Life Cycle
Taenia solium
Pork tapeworm
Taenia Cyst in Muscle
Phylum Nemertea
• Ribbon worms
– Ciliated epidermis
– Locomotion – gliding over slime track and
muscular contraction
– Complete digestive tract – mouth and anus
– Extracellular digestion
– Proboscis to capture prey
Amphiporus