BIOL2009 LT9 Platyhelminthes

Phylum Platyhelminthes

 20,000 extant species of free-living and parasitic
worms – described “triploblastic acoelomate
bilateria”
 Display a wide variety of body forms and are
successful inhabitants of wide range of
environments
 Most flatworms are flattened dorsoventrally, although the
body shape baries from broadly oval to elongate and ribbon-
like – a few bear short tentacles at the anterior end or have other
elaborations of the body surface
 Free-living forms range from less than 1mm to ~30cm long (most are 1-3cm long) –
certain tapeworms attain lengths of several metres
 Medical + Economic impact: blood flukes (schistomiasis) – (No.1 neglected tropical
disease), cattle liver flukes (eg. Fasciola) – economic losses consist of costs of
anthelminitics, contamination, losses in production due to mortality, tapeworm
infection
 Biological Significance/Interest: Regeneration/stem cell biology, maintain
totipotent somatic stem cells, regenerative abilities common among free-living
forms
Phenotypic/developmental plasticity – parasitism vs. symbiosis, single origin of
parasitism

Phylogeny

Bauplan/ General Characteristics

 Combined features of the platyhelminths represent a suite of attributes marking
major advancements in the evolution of the Metazoa, although some recent work
suggests that these animals might have had a coelomate ancestry
 Coupled with a third germ layer (mesoderm), bilateral symmetry and cephalisation,
trend toward centralisation of the nervous system
 Usually hermaphroditic, also dioecious
 Acoelomate: mesoderm gives rise to parenchyma, muscle and reproductive organs
 Digestive system typically blind gut/sac (no digestive system in tapeworms)
 Excretion via protonephridia (aka flame cells)
 Tegument: a ciliated epithelium, except in the parasitic lineages




Class Turbellaria

, Planarian worms – model organism of regeneration research
Morphogenic signalling
- Wnt signalling, APC, β-catenin
- β-catenin cell-signalling pathway very common in the animal kingdom: knowck
down β-catenin, loss of anterior-posterior identification
- Normal worm become hypercephalised – grows new heads

Flatworm “neoblast” – (ie. somatic stem cell)

- Unusual in maintaining somatic stem cells scattered
throughout the parenchyma throughout their lives
- Each cell division results in one new stem cell and one
differentiated cell
- The ability and degree of regeneration differs among
groups (parasitic groups have no regenerative abilities)
- Macrostomum lignano – new model flatworm for research

The Parasitic Worms (Neodermata = Trematoda + Monogenea + Cestoda)

Evolutionary Adaptations

 Evading the host immune system (parasites tend to be host specific)
 Massively increased reproductive output – extreme R-strategists: most effort in
producing large numbers of offspring, little in rearing them and 95% body mass
devoted to reproductive organs
 First vertebrate hosts most likely to be fish

Formation of the neodermis

Digenea (flukes)

 ~18 000 species
 Most diverse group of flatworms
 Primarily enteric (intestinal) parasites of all vertebrate groups
 Have major medical and economic importance
 Highly complex life cycles involving 3-5 hosts
 First host almost always a snail, less frequently bivalves, scaphopods and annelids
 Both active and passive transmission strategies




Human bloodflukes – Schistosoma masoni

 Causes Schistosomiasis/Biharziasis

,  Dioecious –separate sexes
 Blood dwelling
 Life cycle
- Begins with miracidia (free-swimming/active penetration)
– Sporocyst (intra-mulloscan)
– Cercaria (free swimming/active penetration)
– Schistosomula (juvenile worm/active penetration)

Cestoda (tapeworms)

 ~9000 species
 Have no gut/mouth- obtain
nutrients via absorption across the
tegument
 Majority are segmented or
strobilated
 Increase reproductive output via
repetition of reproductive organs
 Enteric parasites of all major vertebrate groups
 Have complex life cycles usually involving arthropod – 1st intermediate host
 First host almost always snail, less frequently bivalves, scaphopods, annelids
 Tapeworm infections most serious when humans play role of the intermediate host
(Taenia sollium causing neurocysticerocosis. Echinococcus multilocularis causing
hyatid disease)

Broad-fish tapeworm (Diphyllobothrium latum)




Monogenea