Download Platyhelminthes: The Flatworms

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Platyhelminthes: The Flatworms
I. Introduction
A. Flatworms are triploblastic acoloemate animals and bilaterally symmetrical
1. So we will see a departure from the non-cephalized, radial symmetry of
cnidarians
B. 18,500 species in 4 classes show taxonomy overhead
1. Turbellaria
2. Trematoda
3. Cestoda
4. Monogenea – won’t cover
C. Turbellarians are mostly freeliving but the latter three classes are exclusively
parasitic
D. Phylogeny – conventionally thought that flatworms ancestors gave rise to
coeloemate animals – but there is now some thought that flatworms have derived
from coelomates - once again illustrates the fluid nature of the field of invertebrate
phylogenetics
II. Common anatomical and life history features
A. Single body opening – no anus
B. Generally increased cephalization – anterior elaboration of nervous system
1. In some advanced species the presence of ventral nerve cords
C. Body shape very flat
1. No specialized respiratory organs ie gills or circulatory system – because of
large surface area to volume ratio gas exchange takes place across this large
surface area
2. Mesodermal layer develops into a loose parenchyma tissue
D. But also to aid is waste processing – is an organ – protonephridia
1. Show fig 8.1
2. Composed of a mesh cup and cilia projecting into a tubule
3. Tubule is long and convoluted
4. Functions in a roughly analogous fashion to our kidney
a) Force water through the mesh. Mesh prevents good stuff like proteins from
escaping into tubule
b) As water moves through tubule, ions are selectively absorbed or secreted
c) Also important function in osmoregulation – water can be pumped out or
pumped into tubule
d) Eventually waste water exits through excretory pore
E. Life history. Majority of flatworms are simultaneous hermaphrodites – at once
females and males – so can mutual exchange of eggs and sperm
1. Outrageous life cycles in many cases
III. Class Turbellaria
A. General characteristics
1. Represent 16% of flatworm species – 12 orders
a) Very complex array of animals that may not be closely related
b) Hot area in molecular phylogenetics
2. Most are freeliving very small – show Barnes diversity
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3. Most are marine, a few freshwater and some terrestrial – in wet environments
4. Most are benthic but some are good swimmers
B. Body Wall
1. Fig 8.4
2. Body covered by a ciliated epidermis
3. No hard covering or cuticle but often worm covered in mucus
a) Gland cells present that secrete rhabdites that swell and form mucus
(1) Used to prevent drying out, sometimes distasteful, locomotion
(2) Show Barnes figure
4. Also have circular and longitudinal muscles – result in coordinated movement
C. Locomotion
1. Many move by secreting a viscous mucus on ventral surface and beat cilia in
this mucus.
2. Along with coordinated muscle contraction this effects movement
D. Gradation of nervous system development show Fig 8.2
1. Acoels have simple nerve net
2. More advanced have cords
3. most have eye spots in head area and these are simple photoreceptors and
statocysts
E. Feeding and Digestion – difficult to generalize about digestion – huge gradation
1. In most cases they are carnivores or scavengers
a) Can catch small prey or feed on sessile prey or eat dead animals
2. Varies from no space – acoel flatworms to many partitions in gut
a) Show fig 8.7
b) Gut type used as a taxonomic character
c) Substantial disagreement about which groups are more advanced – acoels
once thought primitive – now thought to be derived
3. More advanced species have a protrusible pharynx show fig 8.8
F. Reproduction – quite sophisticated –
1. Asexual reproduction – many can divide in half and bud
2. Usually simultaneous hermaphrodites with evolved reproductive structures –
penis, vagina etc – get detail from book if interested
IV. Trematodes
A. Most diverse group of flatworms all parasites; 10,000 species
B. Common endoparasites of all classes of vertebrates
1. Some are debilitating to humans and livestock
C. Morphology –As adults have some resemblance to turbellarians except are not
ciliated – instead have a tegument tough –
1. Much of body given over to reproduction – large amount of space taken up by
gonads and accessory structures show fig 8.16
D. Extraordinary life history – incredibly complex
1. Complete life cycle requires at least two – sometimes more -hosts endoparasitic
a) Sometimes up to 5 different larval stages
b) Intermediate hosts are often gastropod snails
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2. We will discuss the specifics of one species Schistosoma which causes
Schistosomiasis – a disease that kills 800,000 people every year, affects 300
million people worldwide and affects millions of livestock
a) Show fig 8.18 c
(1) Encapsulated embryos hatch from vertebrate feces
(2) Burrow into a snail host and develop into 2nd larval type
(3) 2nd larval type can reproduce itself many times over within snail host
(4) these all develop into 3rd larval type – which can swim
(5) this type breaks out of snail into water
(6) bores through skin and into blood to lung to liver to veins of intestine
(7) migrates to intestine and grows into adult fluke
(8) fluke produces gametes and starts over again
b) symptoms
(1) huge blockages of circulatory systems – causing elephantiasis,
inflamation, necrosis
V. Class cestoda show figs 8.12 and 8.13
A. Like trematodes cestodes are entirely parasitic – all endoparasitic
B. Unlike trematodes they bear very little resemblance to freeliving flatworms
C. Morphology – highly specialized existence in vertebrate guts
1. Nonciliated outer tegument
a) resists digestive enzymes of guts it takes residence in
2. No mouth - absorbs nutrients across body wall
3. Anterior end: scolex – possesses suckers and hooks enabling worm to embed
into host gut lining
4. Neck – below scolex – proglottids bud here
5. Proglottids are series of identical gamete factories – essentially exclusively in
the egg and sperm production business
6. Produces copious amount of eggs and sperm
D. Life histories also complex but won’t discuss here – can get detail in book
1. Can have multiple hosts
2. Eggs can encyst for long periods of time
3. Eventually a larva can hatch out – into an intermediate host
a) Larvae have typical flatworm morphology
b) This stage can also encyst for long periods of time
4. Intermediate host ingested by final host
5. Worm grows into adult form