7: Platyhelminthes

Trace the plathelminthes branch

• Branches of the Metazoa, eumetazoa, bilateria, and protosomia group 

How many species of plathelminths are there? What 2 lineages do they split into?

There are 23,000 species

Catenulida lineage and rhabditiphora lineage

What are examples of the catenulida lineage? What does the rhabditiphora lineage split into?

• Catenulida lineage (100 species) 

  • Ex. chain worms: free-living, microscopic freshwater forms 

• Rhabditiphora lineage (vast majority of species) 

  • Clase turbellaria: free living bottom dwellers of marine and freshwater environments 

  • Clade neodermata: parasitic forms (flukes and tubeworms) 

What were plathelminths originally classified as? What are the lineages classified with pseudocoelomates

• Previossly classed as a “primitive” basal bilateral with acoelmorphs )other acoelomate flatworms) 

  • Justification was comparative anatomy between them

• Other lineages nested within pseudocoelomates (nematodes and rotifers) or coelomates, the true protostomes (mollusks, 

What is the current placement of platyhelminths

Places with spiralians: grouping off of lophotrochoza with a spiral cleavage pattern

Where does spiralia come from? What is lophotrochozoa

Deep within protozoa while acoelomoprhs split before the protostoma

• Lophotrochoza: superphylum of protozoan bilaterians 

  • Lophopore = horseshoe-shaped feeding structure 

  • Trochozoa: top-shaped ciliated larval forms 

  • Not all members perfectly share this, ex. Platyherminthes don’t have a lophore so they can also be called spiralians 

How many smaller phyla are in spiralia

8 smaller phyla

What do acoelomorpha and platyhelminths have in common? what do they differ

Both:

• No coelom or anus 

• Unsegmented 

• No respriatory or circulatory system 

Acoelomorpha

• Blind gut, free living marine worms, simple nervous system w no brain

Platyhelminths

• branched gastrovascular cavity, still simple gut with one opening

• includes free living and parasitic forms

• some have brain and cephalization

What makes platyhelminths Bilateria? What is the mesoderm? How does endoderm form?

• In diploblasts, endoderm formation occurs during gastrulation, embryonic formation of the gastrovascular cavity 

• In triplolasts (all Bilateria), some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm 

• Evolution of the mesoderm allows for more complex body structures and organ systems 

What are the germ layer distributions in flatworms?

• fairly streamlined organ systems 

• Ectoderm -> epidermis, nervous system 

• Mesoderm -> parenchyma (collagen fibers, filling space and providing attachment sites for muscles), well developed muscles

  • Mesoderm is’t giving rise to anything special/novel yet 

• Endoderm -> gastrodermis

Why are flat worms so flat?

• Flat body form due to lack of coelom )fluid filled body cavity separating the gut from the outer body wall) 

  • Flat worms compared to worms that have coeloms, coelom is fluid filled and has hydrostatic function where it pushes against the busy wall to keep shape 

What are the advantages of a flat body

• We can deduce that the platyhelminths might have had a coelom and lost it, or it could have never evolved a coelom at all 

• Flat body is slay for gas exchange (surface area volume relationships) 

• Invest as much in thermoregulation 

How are platyhelminths digestive system’s designed? What type of gut do they have?

• Flatworms have an incomplete gut, lacking an anus 

• Pharynx (located on the underside of the animal) doubles as a site of eating food and excreting waste 

• Some have evolved highly branched gastrovascular cavities, which are more efficient for dibesting and distributing nutrients 

  • Have lots of surface area in it for digesting and obtaining nutrients 

explain the platyhelminths nervous system

• simple nervous system

• Kind of cephalized because mouth/feeding apparatus not located on head 

• Nerves concentrated at the interior end (cerebral ganglia) could be interpreted as primitive brain 

• Pair of nerve cords run from brain to tail in ladder like fashion 

  • Simplest arrangement of a nervous system in bilateria 

  • Very effective for them 

• Eye spots 

• Chemoreception (auricles) used to detect food 

How do platyhelminths have such flexibility? What are the three muscle layers and what movement does each of them handle? what is important about the platyhelminths muscle tissues

• Well developed muscles compine with lack of internal structure to facilitate near infinie range of movement 

• Three distinct muscle layers 

  • Circular: causes flattening of the body 

  • Diagonal: facilitate bending and twisting 

  • Longitudinl cause elongation of the body 

• Muscle tissues guidepost cells, which guide body regeneration 

What are examples of free living flat worms? What do they eat? Predation? Abilities?

• Examples: planarians, hammerhead worms, marine flatworms 

• Generalist scavengers and predators - eats anything small enough to ingest 

• Ones that are big enough to see can be fearsome predators 

  • Can sense prey trails 

  • Flatworm slime can contain a fast active anesthetic 

• Range from awaiting to moist terrestrial habitats 

• Impressive regeneration capabilities 

When does parasitism first occur? did ectoparasites or endoparasites appear first

• despite ancient origins, parasitism is relatively new to flatworms 

• Fossils don’t appear until the time large vertebrates exist 

  • Around devonian 

• Ectoparasites appeared before endoparasites, but still they didn’t rely show up until large animals showed up 

What is the working hypothesis for why parasitism developed and how we went from ectoparasites to endoparasites

• Having hosts to parasitize 

  • Large vertebrates is important tomaking parasitism even like valuable 

• Ectoparasitism -> endoparasitism 

1. Diversifying from simple skin parasites into specialied ectoparasites (ex. Living in gills, eye, eventually buccal and branchial cavities) 

2. Endoparasitis and complex life cycles came later 

What is the old working theory for transition from ectoparasites to endo

• transition likely occurred

• Ectoparasites giving rise to endoparasites 

• Endoparasites split off from ectoparasites 

  • They originated from a single common ancestors  once

What is the modern day transition of parasitism theory

• Origin of parasitism split and two different options could have occurred 

• Ectoparasitism evolving then endoparasitism and then reversion possibl

• Two branches splitting off and ectoparasitism and endoparasitism evolved at the same time 

What are some preadaptations for endoparasitism

• Flattened body for efficient absorption in nutrient rich environment 

• Simple digestive system can be modifed to a generalist fiet 

• High fecundity -> populations can expand rapidly within a host 

What is the endosymbiotic envrionment? What does it mean for the evolution of endoparasites? Why are there different host environments? What are the two types of hosts

• Nutrient rich -> simply digestive system 

• Stable internal environment -> lose unnecessary sense organs, locomotory structures 

  • Outside worms have to worry about temp regulation, osmotic balance, predators 

• Different host environments preferred for different functioning -> complex life cycles separating functions into different hosts 

  • Intermediate hosts: where growth occurs, numerous, easy to colonize 

    • Often something like a mollusk

  • Definitive hosts: where reproduction occurs, large, resource rich 

What are the major 2 lineages of endodermic parasites? What are the differences between them? Suckers, guts, segmentation, infection sites

• Tematodoa (flukes) 

  • Non-segmented and leaf shaped 

  • Maintain simple gut 

  • Have specialised suckes 

  • Generally infets the bie ducts, liver, or blood 

• Cestoda (tapeworms) 

  • Segmendted in a sense

  • Have no mouth or digestie system 

  • Absorb host nutrients through diffusion 

  • Infects the intestines 

Flukes: explain their suckers, what do they like for their definitive hosts, what is their link to adaptive radiaion, What is interesting abot luechochlodirium paradozum?

•  have two specialized suckes (one oral, one ventral) used to attatch to host 

  • Intermiediate hosts (where growth occurs) are typically mollusks 

• Definitive hosts (where reproduction occurs) are typically vertebrates 

• Most flukes tend to specialize on particular host species 

  • Can lead to adaptive radiation 

• Luechochlodirium paradozum larvae infect eye stalks of snal host to enhance transmission 

  • Can affect hosts to get themselves more likely to be eaten by birds 

Explain fluke infections in humans and pets: how do we get infected, what animals are they common in, what is schistosomiases

• How do we get infected 

  • Eating raw or undercooked fish, crabs, shellfish 

  • Bathing in contaminated water 

• Liver flukes are common in cats, dogs, and livestock 

  • Severe infections can cause liver failutes 

• Schistosomiases = common infectionin tropical regions 

  • Affects urinary tract and intestines 

  • Chronic infection leads to enlargement of liver and spleen, UT complications, anaemia, increased risk of blaffer cancers 

Tapeworms: what functions have they lost, what are proglottids, how do they grow, what are their common hosts

• Having secondarily lost mouth and gut- absorb nutrients through the skin 

• Proglottids: long ribbon like repeated packets (segments) containing male and female reproductive organs 

  • Grow by budding within the intestine; each proglottide eventually breaks off and releases eggs 

• Also show high host specialization 

  • Definitinve host = always a vertebrate 

  • Intermediate host may be vertebrate of mollusk 

What are tapeworm infections like in humans

• Causes malnutriona and anemia 

• Infections can be long lasting (can last 30 years and grow up to 30ft) 

• Maria callas (1923 - 1977) opera sincer who lost 30kg after eating raw stake 

  • Spawned mythical tapeworm diet pill