Phylum: Platyhelminthes

Phylum: Platyhelminthes

1. Triploblastic invert, mesoderm between outer ecto and inner endoderm, Acoelmates= primitive triploblastic inverts that lack body space other than gut

2. Distinguishing features: 1. acoelomate 2. soft-bodied animals 3. bilaterally symmetrical 4. obvious head end

   • most have mouth at anterior, some in centre of body. series of tubules act as excretory system= flame cells (protonephridia). maintains movement of fluid and removes waste through excretory pores, also removes ions and water (osmoregulatory)

3. Hermaphropdites, divided into 4 classes- Turbellaria, Monogenea, Trematoda, Cestoda

   • Free living Turbellaria- aquatic & benthic, darkly coloured to brightly coloured (some marine), ciliary movement- gliding to muscular creeping, pigment cup eyes head and body, 2-30 (detect light/shade), predatory- soft bodied invert/dead animals. swallowed whole or digestive enzymes aid
     • Repro- diff objectives driving force in copulatory mechanisms, herm have sexual conflicts= physically damaging sex
       • inject sperm under partners skin directly to eggs, females bear burden of wound healing/ lose control over fertilisation.
         • Penis fencing- show strong avoidance when struck, 2 individuals interact if both are sexually active. first to inseminate obtains longer injection time= father more eggs, less wounds to heal. asymmetrical outcome of insemination favours first stabber- better genes to pass on
       • Pseudoceros bifurcus- controlled repro, penis fencing an attempt to increase benefits of sperm donation over cost of sperm receipt. Fuels evolutionary arms race between strike and avoidance behaviour.
   • Majority of platyhelminthes are parasitic, host specific, internal organs to external surfaces

4. Trematoda/Flukes- blood, intestinal, liver. Aspidobothria (evolution in action, shows transition freeliving/parasitic, Dignea= obligate parasites, complex life cycle with 1/2 intermediate host for a sexual repro

   • Eusocial behaviour- 1/2 repro individuals, morphologically different within colonies. after invasion- divide a-sexually to form colonies genetically identical. soldiers dont repro (2% size of repro flukes) eggs ingested through faeces, miracidium invades
     • Ribeiroia ondatrae- causes deformities in frogs, miracidia ingested through faeces by aquatic snails. life cycle complete when definitive host consumes frog. metacercariae cluster around pelvic girdle and create road block, disrupts limb growth- increases chance of being consumed if no legs.
       • tadpoles exposed to atrizine more vulnerable to Ribeiroia- chemicals leached into water systems

5. Cestoda/Tapeworms. larval stage of tapework=bladderworm

   • endoparasitic adults of carnivorous verts, hermaphrodites, acoelomate.
     • scolex (1mm) equipped with suckers/hooks for attaching to gut wall, short non-segmented neck, series of proglottids make up strobila region
       • repro- self fert can take place when strobila folded back, gonads mature at dif times. segments full of mature eggs, terminal proglottid breaks from strobila & passes out with faeces.
         • intermediate host needed-metacestode develops-ingested by host-eggs hatch, release onchosphere-larva carried to development site by blood. grouped by budding- protoscolices form on germinal membrane, produce another worm if host is suitable
           • non-budding= Taenia solium Budding= Taenia multiceps
           • increase chances of predation by intermediate host- parasite alters behaviour of host