Acoela (Flatworms)

Order Acoela (Flatworms)

• Bilateral symmetry
• Cephalization: Differentiation of head
• Neural ganglia: cluster of nerves at the back of the head (functions as brain)
• Less developed in sedentary animals
• Triploblastic
• Mostly marine worms
• Acoelmates: do not have a body cavity
• Flat
• Very minimal cephalization
• Do not have:
• Brain
• Anus
• Gut cavity
• Uses endocellular digestion: food is digested in vacuoles
• New phylogeny suggests Acoela is sister to Deuterostomia
• Clade Lophotrochozoa
• 18 Phyla
• Simple to complex worms
• Lophophore: feeding structure
• Trochozoa: larval stage

Phylum Platyhelminthes

• Flatworms
• Acoelmates
• No fluid filled cavity
• Just solid tissue
• Triploblastic
• Breathe using diffusion
• No specialized circulatory or gas-exchange system
• Thin skin
• Not a high surface area to volume ratio
• Incomplete digestive tract
• Has mouth and gut, but no anus

Class Turbelleria

• Free living flatworms
• Marine and predatory
• Epidermis covered in cilia
• Pair of ventral nerve cords
• Pair of ganglia
• No anus
• Food that is not digested is ejected from the mouth

Class Cestoidea

• Mostly parasites
• Endoparasites: Parasitize another organisms digestive system
• Mostly vertebrate hosts
• Invertebrates are intermediate hosts
• No digestive system
• Scolex: Suckers / hooks that hold onto hosts gut

Tapeworms

• Proglottids: long chain of units devoted to reproduction
• New ones produced near scolex
• Filled with eggs
• Also absorbes nutrients
• Creates many
• These fall off to infect other hosts
• Complex life cycle

• Needs 2 types of host to complete
• Intermediate Host (IH)
• Usually invertebrate
• Consumes the eggs
• Eggs hatch into larvae which encyst: attach themselves inside host
• Parasite goes through asexual reproduction
• Definitive Host (DH)
• Usually vertebrate
• Eats the IH
• Larvae develop into adult tapeworm inside DH
• Parasite goes through sexual reproduction
• Tapeworms in humans
• Comes from
• Undercooked pork and beef
• Raw fish
• Can cause blockage and nutritional deficiencies

Class Trematoda

• "Flukes" with 2 large suckers
• Blood flukes (Schistosomia)
• Causes "Swimmers Itch"
• Have complex life cycles