Phylum Platyhelminthes Flashcards
Platyhelminthes
Flatworms • bilateral symmetry and cephalization • triploblastic • organs • mesenchyme (mesoderm) • a coelomate • 34000 described spp. • 4 classes -1 free-living (Turbellaria) -3 parasitic (tapeworms and flukes) • 80% parasitic - probably more • 36,500 undescribed? • no special systems for respiration or circulation - body surface • <1 mm to 30 m long • sac-like gut, no anus • biflagellate spermatozoa (most) • hermaphroditic (most)
Nervous system
– more centralized &
complex than Cnidarians
– 2 main nerve trunks (ladder- like)
– brain in anterior (fig. 8.3) – sensory structures:
• Touch
• Chemoreception (smell) • Photoreception
Protonephridia
Flame cells Beating cilia create negative pressure. Wastes filtered, ions absorbed, water balanced, excretion of ammonia, urea Most common in freshwater species
Reproduction
- copulation & internal fertilization
- almost all: simultaneous hermaphrodites
- unlike Cnidaria, many sex organs, glands, etc.
- many Turbellarians: asexual reproduction by transverse fission
- also traumatic insemination, hypodermic impregnation
Class Turbellaria
Free living flatworms
Simultaneous hermaphrodites
Most Turbellarians lack a larval stage
• Several marine species have a Müller’s larva
Neodermata
Cestoda, Monogenea, & Trematoda (Parasitic)
Form a monophyletic group
Epidermis:
- cilia absent in adult (lost)
- syncytial tegument - many nuclei, no cell
membranes
Class Cestoda
(Tapeworms)
• 3500 spp
• endoparasitic in vertebrate digestive tract • body covering
-tegument - syncytial, highly folded • holdfast
-scolex
• subdivided body
-proglottids
No digestive tract
• nutrients absorbed directly across body surface from host’s gut
– Active transport & diffusion
Microtriches
Outfolding of tegument
Increase surface area
Proglottids
- for sexual reproduction
– bud from neck, several / day
– each has male & female reproductive systems – ~ 50,000 eggs / proglottid
– fertilized by sperm from:
• Different individual
• Same individual, different proglottid • Same proglottid
– mature (gravid) proglottids at posterior - segments “like an ephyra” more than an annelid
Scolex
Holdfast w/ hooks, suckers
• neck (site of proglottid budding)
• complex structure, diversity of forms
Cestode life cyle
Nearly all spp need 2 hosts
Nearly all spp need vertebrate host to mature
Most take advantage of predator-prey relationships
(no need to seek hosts)
Taeniasis
Mostly mild symptoms including nervousness; insomnia, anorexia, weight loss, abdominal pains and digestive disturbances;
Most infections are asymptomatic.
Tapeworms and people
• Typical infection is with adult worm
• Less common, but sometimes worse, is infection with cysticerci due to consumption of oncosphere larvae
• Leads to cysticercosis - can be benign to crippling
• (paralysis, blindness)
Handful of species infect humans; 4 + genera
3 species of Taenia mature only in humans we are the only definitive (final) host :
Taenia saginata - intermediate host: cattle Taenia solium - intermediate host: pigs Taenia asiatica - intermediate host: pigs
Tapeworm evolution
Traditional hypothesis: Taenia spp. “jumped” into
humans from cats, dogs, pigs etc, ~ 10,000 years ago
Closest relatives of the Taenia in humans are Taenia in wild African carnivores. Not domestic spp.
“Our” tapeworm species 0.78- 1.7 million years old. Not 10,000 years old.
(started in Homo habilis prior to leaving Africa)
If correct, then we infected our domesticated species!
Monogenea
• ectoparasitic
• skin or gills of ectothermic vertebrates
• frogs, fish
• no intermediate host • 0.03-20 mm
• holdfasts
Prohaptor
Haptor/opisthaptor
• Most “graze” on host mucus and skin
• Hermaphrodi
• Name - means “one generation” • Highly host & niche specific
• ~ 11,000 species named
• ~ 20,000+ species total?
• Some spp. in bladders of turtles and frogs
• 1 species in eye of hippo!
• Usually not a threat to fish except in hatcheries
Flukes
Monogenea and Digenea
Prohaptor
Anterior
-suckers and adhesive glands
Opisthaptor
Posterior
-suckers, hooks and sclerites
Gyrodactylus sp.
Parasite of freshwater and marine fish
Larvae develop internally, 3-4 inside each other - ie the offspring are born pregnant with pregnant offspring inside them ! = polyembryony
- causes populations to grow quickly
- one individual can grow to thousands in days
- problem for fish farmers, wild populations less affected
- has been implicated in extinction of many populations of Salmon (from rivers)
Class Trematoda
Flukes, blood flukes
• Endoparasites of vertebrates, molluscs, etc.
• ~ 10,000 species (~5,000 unnamed)
• Two subclasses:
– Digenea >95% of species
– Aspidogastrea 32 - 80 spp. • Synapomorphies:
– Posterior sucker
– Life cycle with molluscs & vertebrates
Subclass Digenea
Largest group of trematodes • >=1 intermediate host (prior to final) • 2 or more hosts total - up to 5 • free-living larval stages search actively for host (not like cestodes)
Digenea life cycle
- miracidium larva - ciliated, free-living, ( host 1), 3000/day
gutless - sporocyst - cilia lost, no mouth, in host, nutrients by absorption, produce more sporocysts or produce redia
- redia - with mouth, into digestive gland or gonads
- cercaria - leave redia & host, seek 2nd intermediate host 5. metacercaria - encysted waiting stage (for final host)
- adult
Endoparasitism - Problems to overcome
“Finding” the final host
Increase reproductive effort
1. High fecundity
2. Asexual reproduction & self fertilization
- Self fertilization does not produce clones of the parent (why?)
3. Short generation time
Increase potential contacts
1. Use intermediate hosts
Affect intermediate host behaviour
Active vector
2. Encyst - wait patiently (metacercaria)
