Platyhelminthes (Flat worms)
I. Introduction
a. Now we talk about Bilateria - animals with bilateral symmetry
b. Platyhelminths have no coelom
1. Acoelomate
c. Triploblasic - three germ layers
1. As embryo develops there are three clear layers
a. Ectoderm - forms epithelium and nervous system
b. Mesoderm - forms internal organs (muscles, reproductive organs)
c. Endoderm - forms digestive tract
d. Cephalization - sensory organs and nerves concentrated on anterior
II. Phyllum Platyhelminthes
a. Replacement of Cnidarian mesogloea with parenchyma derived from mesoderm
1. Cells fill interior spaces
b. Flatworms vary in size millimeters to many meters
c. Free-living and parasitic
1. Free living in Class Turbellaria
III. Form and function
a. Cellular, ciliated epidermis - move by ciliary action
1. Rhabdites - swell and forma protective sheath when animal threatened
b. Nutrition and Digestion
1. Mouth and pharynx
2. No anus
c. Excretion and osmoregulation
1. Protonephridia - simple excretory organs
a. Flame cells - have cilia that keep waste moving down tubule to outside
b. Complex series of tubules with flame cells.
d. Nervous system
1. Ladder form - longitudinal nerve cords
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Platyhelminthes (Flat worms)
2. Cerebral ganglia
3. Sense organs
a. Cephalization
b. Ocelli - sensitive to light
e. Reproduction
1. Great powers of regeneration
a. Animal can split in two to make 2 animals
b. Can be sliced to regenerate
2. Monoecious - both male and female
a. Lay eggs
1. Some have food for embryo in egg cell
2. Some provide extra yolk cells for the embryo - enclosed in an
eggshell
b. Testis
c. Develop both male and female organs during breeding season
1. Copulation
2. Eggs and yolk cells enclosed in small cocoons that are placed under
stones and plants
3. Juveniles resemble adults
IV. Class Turbellaria
1.Free-living
2. Planaria
V. Class Trematoda - Flukes
1. Chinese Liver fluke is an example - Class Digenea
a. Most flukes have snails as one host - vertebrates as other
a. Sometimes a couple of hosts
b. "Final" host is the one in which sexual reproduction occurs
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Platyhelminthes (Flat worms)
b. Chinese Liver fluke life cycle
1. Shelled embryo passes in feces of final host
2. Hatches into miracidium - ciliated larva - penetrates snail
3. Develops into sporocyst - reproduce asexually to make lots of
sporocysts
4. Some sporocysts develop into rediae
5. Rediae develop into cercariae
6. Cercariae emerge from snail and penetrate 2nd intermediate host fish
a. Cercariae swim around to find fish
7. Metacercariae encyst in muscles of fish and get eaten by humans
8. Form fluke in liver - live in bile ducts
a. Very painful
b. Diagnose with fecal examination
c. Schistosoma - Blood Flukes - Schistosomiasis
1. Similar to liver fluke
2. Dioecious - males and females
3. Eggs discharged in human feces or urine
2. Hatch to miracidia - need to find a snail fast
3. Sporocysts in snail
4. Sporocysts make cercariae directly
5. Penetrate human skin
6. Develop in liver
1. Move to species specific sites in the body
2. Eggs through gut or bladder lining
3. Many eggs don't make it and cause infections
4. Very painful
VI. Class Cestoda - tape worms
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Platyhelminthes (Flat worms)
1. Very different from preceeding classes
a. Scolex
b. Proglottids
2. Life Cycle : Taenia saginata
a Proglottids break off and pass with feces
1. Proglottids attach to vegetation
2. Get eaten by cattle
3. Embryos live on grass for as long as 5 months
b. Shelled larvae (oncospheres) - burrow into intestinal wall and reach muscle (meat)
1. Encyst in muscle
c. When meat is eaten by host - cyst wall dissolves - scolex attaches to intestinal wall
1. 2 to 3 weeks get mature worm
d. Book says that tapworms are fairly common
1. Killed by thorough cooking of meat
3. Lots of other nasty Platyhelminth parasites
VII. Phylogeny
1. Origin of bilateria?
a. Planula larva of Cnidaria
2. Bilateral symmetry is an advantage for creeping or swimming animals
a. Sensory structures concentrated on anterior end
3. Adaptive radiation
a. Somehow predisposed to be parasites
b. Radiated as parasites
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Platyhelminthes (Flat worms)
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