bio 205 chapter 8: the platyhelminthes

phylum platyhelminthes

approximately 20,000 species of flatworms
first bilaterally symmetric animals, to exhibit cephalization, triploblastic, and to possess muscle tissue from mesoderm
organ systems
acoelomates
incomplete gut

organ-systems

free-living flatworms
ganglia with nerve cords, incomplete gut with an intestine protonephridia with flame bulb cells

ganglia with nerve cords

nervous system made up of ganglia, ventral (longitudinal) nerve cords, transverse commissure and resembles a nerve ladder

ganglia

pockets of nerve cells found within the head

ventral (longitudinal) nerve cord

extends posteriorly from each ganglion

transverse commissure

nerves that connect the 2 ventral (longitudinal) nerve cords

incomplete gut with an intestine

consist of a mouth and pharynx

mouth

positioned on the ventral surface at the end of the tube-like pharynx

pharynx

leads to an intestine where food will be digested and nutrients absorbed
nutrients diffuse through body tissues, no circulatory system exists

acoel

lacks a true intestine

rhabdocoel

an elongated intestine with an anterior and a posterior chamber

alloeocoel

an elongated intestine with folded walls

triclad

an intestine with 3 branches (one anterior and two posterior)

polyclad

an intestine with many branches throughout the body

protonephridia

represent an excretory system that removes waste from the body tissues

flame bulb (flame) cells

possess cilia
waste enters the cell as cilia waves and is moved into a tubule and out a nephridiopore on the outer surface

regeneration

the replacement of lost body parts, studied greatly in planaria

class turbellaria

free-living flatworms (planaria)
possess all major organ systems
freshwater and marine
arrowhead shaped head → auricles
ocelli
covered in cilia

sperm duct

also known as the vas deferens, tube that connects to seminal vesicle and leads sperm to it

seminal vesicle

where the sperm are stored

male reproductive steps

sperm duct/vas deferens → seminal vesicle → penis → gonopore → copulatory sac/seminal receptacle

oviducts

tube that connects to the vitellaria/yolk glands where eggs receive nourishment

vitellaria/yolk glands

eggs receive nourishment

female reproductive steps

oviduct → vitellaria/yolk gland → gonopore (fertilization) → gonopore

class trematoda

parasitic flukes that are parasites to vertebrates
infect an organ of the digestive system
true and intermediate hosts

true host

harmed by the parasite

intermediate host

not harmed by the parasite, used for maturation or for asexual reproduction of larval stages

trematode larval stages

fertilized eggs → miracidium larva → sporocyst → rediae larvae → cercariae larva → metacercarial cyst

miracidium larva

ciliated and targets an intermediate host which is usually a species of snail

sporocyst

transformed from miracidium larva within the snail

rediae larvae

sporocyst undergoes asexual reproduction

cercariae larvae

what rediae larvae develop into and are released from the snail, equipped with a whip-like tail and swim to find a new host

metacercarial cyst

a cercaria larva encysts on something that the host would eat

Opisthorchis (Clonorchis) sinensis life cycle

Chinese liver fluke, live in biliary ducts of human liver
1) fertilized eggs are released out with the human’s fecal matter
2) eggs hatch into miracidia larvae within the water and swim to a snail
3) each miracidium forms a sporocyst in the snail
4) sporocyst gives rise to rediae larvae and then cercariae larvae
5) cercariae larvae then leave the snail and swim to a fish, usually a carp or related species, where it then harms the fish
6) cercariae larvae form metacercarial cysts on the skin or gills of the fish
7) transferred to a human and develop into adult worms if the human eats the infected fish

Fasciola hepatica life cycle

sheep liver fluke, adult worms live in the biliary ducts of a human liver (true host) and intermediately in a snail
1) fertilized eggs are released out with sheep’s fecal matter
2) eggs hatch into miracidia larvae within the water and swim to a snail
3) each miracidium forms sporocysts in the snail, which give rise to rediae larvae and cercariae larvae
4) cercariae larvae leave the snail body and swim to nearby aquatic vegetation
5) cercariae larvae then form metacercarial cysts on the aquatic vegetation
6) vegetation is eaten by the sheep and metacercarial cysts would develop into adult worms
7) if a human consumes the infected animal, which can also be a cow, they will become infected

Schistosoma mansoni life cycle

blood flukes that cause schistosomiasis, adult worms live inside the blood vessels of the small intestinal wall
1) fertilized eggs are released out with the human’s fecal matter
2) eeggs hatch into miracidia larvae within the water and swim to a snail
3) each miracidium forms sporocysts in the snail
4) sporocysts give rise to cercariae larvae, and no rediae larval stage exists
5) cercariae larvae leave the snail and swim to a new human host, and no metacercarial cysts exist
6) cercariae larvae penetrate the skin of the human, usually on the skin on the foot or the ankle
7) become new adult worms once they arrive at the small intestine

class cestoda

parasitic tapeworms, usually to vertebrates
lack a digestive tract and ganglia
only longitudinal nerves run through body
scolex and proglottids

scolex

the anterior end of a tapeworm, NOT a head
possesses suckers and possibly hooks for attachment to host’s small intestine wall

proglottids

reproductive units that sit behind the scolex

tapeworm gametes

hermaphrodites
testes produce sperm → sperm duct → vas deferens → gonopore → another proglottid through gonopore → vagina → seminal receptacle
ovaries produce eggs, receive nutrients from vitellaria (yolk gland) and protective shell from shell gland, eggs stored in uterus

tapeworm life cycle

1) gravid proglottids break off and are passed out with host’s fecal matter (gravid proglottids contain hundreds of fertilized eggs)
2) vegetation contaminated with eggs can then be consumed by an herbivore
3) ingested eggs develop in oncospheres and travel through the bloodstream to muscle tissue
4) oncospheres develop into cysticerci (bladder worms) within the muscle tissue of the animal
5) if humans eat raw or undercooked meat from those animals, the cysticerci will become adult worms within the human