Lecture 7: Flatworms

Phylum Platyhelminthes

• flatworms, flukes, & tapeworms

• tripoblastic: ectoderm, endoderm, mesoderm tissue layers

• Organ level of structural complexity

• acoelomates

Gastrulation

early stage of animal developement when embryo folds inward & forms basic tissue layers

Non parasitic worms are part of which group ? What are some of their characteristics?

Phylum Platyhelminthes

Class Turbellaria

ganglia

clusters of nerve cells (mini brains) located at front end (head)

Planarians

Phylum Platyhelminthes, Class Turbellaria, Order Tricladida

• Ciliated epidermis = glide on mucus

• highly regenerative = can regrow lost parts

• nerve net + ganglia

• 3 branched gastrovascular cavity = spread nutrients through body

Flukes

Phylum Platyhelminthes

Class Trematoda

• multiple hosts

• internal parasite w/ hooks/suckers

Class Monogenea

• One host

• External parasites w/ hooks/suckers

Tape worms

Phylum Platyhelminthes, Class Cestoda

• internal parasites within vertebrate intestines

• no digestive tract → absorb nutrients

• scolex (hook & sucker head) + proglottids (hermaphroditic body)

• intermediate hosts (pig → human)

cross section of planarian

• Pharynx = feeding tube

• Intestine = branched gut

• Nerve cord = movement & stimuli

• Rhabdite cell = make slime for gliding

• Dual gland adhesive organ = sticking & unsticking

a cross section is always going to be a _______ cut

transverse

cells that create planarian movement

• Parenchymal muscle fibers = shape & hardness

• Longitudinal muscle = contract body to pull in

• Circular muscle = elongate body

• Basement membrane = support

• Epidermal cell = rhabdite cells make slime

• Cilia = beat in waves to glide in slime

Dual gland adhesive organ cells

1. Paired viscid cells = produce adhesive that attach anchor cells to surface

2. Anchor cells = allow subtle muscle contractions to pull forward

3. Releasing cell = secretes chemicals that release attachment

Tegument

syncytial epidermis covering of parasitic flatworms → protects worm + absorbs nutrients

_______ is a defining trait of the clade Neodermata (parasitic flatworms) in the Phylum Platyhelminthes ?

Tegument

Protonephridial system

maintaining osmotic balance by pumping out excess water

1. Flame cells have flagella that beat which pulls water & waste into tubule system

2. Water flows through tubules → collecting ducts → nephridopore (exit pore) → out of body

   • Interdigitation: 2 cells’ membranes interlock like fingers to form a mesh filter (weir) to keep big particles out

Digestive system of a planarian

1. Pharynx = sticks out to grab food

2. Food goes to Pharyngeal chamber → intestine

3. Intestine is branched (diverticula) to spread nutrients through body

4. waste goes back out through mouth

Subepidermal nerve plexus

net of nerves under the skin , no major nerve cords

• simplest system

subepidermal nerve plexus + longitudinal nerve cords

1-5 nerve cord pairs under the muscles

cerebral ganglion = primitive brain collecting/sending sensory info

nerve cords + primitive brain

1-3 pairs of nerve cords, with connecting nerves left & right (ring commissures) forming a ladder pattern

• has an auricle: sensing environment

what sensory types make up Auricle?

1. Tactile receptors = touch

2. Chemoreceptors = chemicals

3. Rheoreceptors = water currents

4. Statocysts = gravity

5. Photoreceptors = light/dark