BIOL 4105 Chapter : 6 26 12 Platyhelminthes
BIOL 4105
6.26.2012
Part II
HELMINTHES
Helminthes: worms
2 groups:
1. Platyhelminthes- trematodes and sestodes. Flatworms.
2. Nematodes- roundworms
I. Flatworms- Platyhelminthes
a. Physical characteristics
i. Flattened dorsoventrally
ii. Bilaterally symmetrical
iii. Definitive Anterior and posterior ends
iv. Acoelomic- NO BODY CAVITY.
v. Brain does not exist in Platyhelminthes.
vi. Obligate parasites. No free-living trematodes or sestodes.
vii. No circulatory, respiratory, skeletal systems.
viii. Trematodes have an anterior end with an oral cavity that allows them to take in nutrients with a
primitive tract ending in a blind sac. This is absent in cestodes.
ix. Fairly well-developed nervous system- nerves. No CNS or PNS- just nerves, muscular system,
excretory system.
x. Platyhelminthes reproduction- monecious- literally hermaphroditic in most trematodes with one
notable exception. There is no exception in the sestodes.
1. Diecious in schistosomes only.
II. Flukes
a. Latyhelminthes
i. Trematoda
ii. Digenea
iii. Digentic trematodes: flukes
iv. Development requires a minimum of two hosts. The first is always a snail. Found as parasites in
all classes of vertebrates.
1. May have 2-4 intermediate hosts between snail and vertebrate hosts.
v. General life cycle
1. Egg→miracidium→sporocyst→redia→cercaria→metacercaria→adult
2. Those in bold are always there, the rest may or may not be present.
3. Egg- operculum, hatches into water, freezing or drying is fatal.
4. Miracidium- ciliated, free-swimming larvae
a. Viable for a few hours
b. Penetrates into a snail in 30 minutes.
c. Chemical homing device- snail mucus trail attracts
d. Some alternate mode- must be eaten by the snail to infect, for some species.
Document Summary
Flatworms- platyhelminthes: physical characteristics, flattened dorsoventrally, bilaterally symmetrical, definitive anterior and posterior ends, acoelomic- no body cavity, brain does not exist in platyhelminthes, obligate parasites. No free-living trematodes or sestodes: no circulatory, respiratory, skeletal systems, trematodes have an anterior end with an oral cavity that allows them to take in nutrients with a primitive tract ending in a blind sac. No cns or pns- just nerves, muscular system, excretory system: platyhelminthes reproduction- monecious- literally hermaphroditic in most trematodes with one notable exception. There is no exception in the sestodes: diecious in schistosomes only. Flukes: latyhelminthes, trematoda, digenea, digentic trematodes: flukes, development requires a minimum of two hosts.
