Download IB104 - Lecture 4

IB104 - Lecture 4 - Worms
Reading - Chapter 25.2, 6, 10, 11
1. The rest of the animal phyla are all bilaterally symmetrical, and are
clearly a monophyletic clade. They are therefore referred to as the
Bilateria, which might be considered a “subkingdom”. All Bilateria also
have three body layers or are triploblastic, with the mesoderm forming
between ectoderm and endoderm, yielding muscles and eventually
bones.
An older, “progressive”,
view of animal
phylogeny in which the
presence of a body
cavity lined with
mesoderm, known as a
coelom, was considered
to be a defining
characteristic of a large
group of animals
including ourselves
(Chordates).
Chordates
Echinoderms
Arthropods
Annelids
Mollusks
Coelomate
Ancestry
Rotifers
Roundworms
Bilateral
Ancestry
Flatworms
Radial
Ancestry
Multicelled
Ancestry
Bilateral symmetry
Radial symmetry
Arthropods
mouth
coelom
segmented
body
Annelids
Mollusks
head
Classical animal phylogeny: based on coelom
chordates
echinoderms
arthropods
tardigrades
annelids
mollusks
no coelom
unsegmented
body
Flatworms
mouth
flatworms
Acoelomate animals
Animals with a 3-layer embryo
cnidarians
saclike
gut
Protostomes
rotifers
Pseudocoelomate animals
roundworms
complete
digestive
system
radial
symmetry, no
cephalization
Deuterostomes
Coelomate animals
false
coelom
bilateral
symmetry,
cephalization
Placozoa
Sponges
Single-celled,
Protista-like ancestors
Roundworms
anus
Cnidarians
Cnidarians
sponges
placozoans
Animals with tissues
Animals
Fig. 25-7a, p. 407
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Protostomes (the phyla today and Wednesday): literally “first mouth”,
because the mouth forms from the blastopore which is formed during
gastrulation, while the anus forms from a secondary invagination at the
other end of the body, which then connects with the rest of the gut,
developing from the archenteron (the space enclosed by endoderm).
Deuterostomes (Echinoderms and Chordates including us - Friday):
literally “second mouth”, because the blastopore forms the anus, while
the mouth forms from a secondary invagination.
Alternative: Phylogeny based on DNA and proteins Deuterostomes
chordates
echinoderms
arthropods
tardigrades
Ecdysozoa
roundworms
Protostomes
rotifers
mollusks Lophotrochozoa
annelids
flatworms
Animals with a 3-layer embryo
cnidarians
sponges
placozoans
Animals with tissues
Animals
blastopore
Fig. 25-7b, p. 407
2. Phylum Platyhelminthes: The ±15,000 flatworms are so-named
because they are flat. They have a clear layer of mesoderm between the
ectoderm and endoderm, and this layer of cells gives rise to various
internal organs including an excretory system and muscles. They have a
well-developed nervous system, including a concentration of sensory
systems for touch, smell, and even primitive sight, at the anterior end this is called cephalization. This is part of being a mobile animal that
navigates across surfaces and encounters things with it’s head first.
Platyhelminthes have a blind
gut with no anus, and they also
have no body cavity and no
circulatory system, hence
breathe through their body
wall, which is only possible
because they are flat. In the
absence of a coelom-like body
cavity they are called
acoelomates.
3. All of these features are most obvious in one of the three major
classes, the Turbellaria or planarians. These are free-living animals,
mostly marine but some freshwater, that eat small animals and decaying
flesh. They have a extrudable pharynx that opens into the blind gut.
brain
nerve cord
ovary
branching gut
pharynx (protruded)
protonephridia
testes
oviduct genital pore
penis
Note - no circulatory system
and no coelom – ancestral or
secondarily lost?
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Land planarian
5. The second class are the Trematoda or flukes. These are all parasitic
worms, commonly with complicated lifecycles, some of which infect
humans. The most serious are the genus Schistosoma, which cause
schistosomiasis or bilharzia. These tiny worms infect and grow in
freshwater snails, and then can enter human skin, travel to the intestine
and bladder to reproduce in large numbers, and then complete the
lifecycle by exiting via urine or feces back into freshwater. Chronic
infection results in damage to the liver, intestines, and bladder, largely
through the body’s own defensive reactions to the Schistosoma eggs.
These are major parasites in the developing world (estimated at 200
million cases per year), and generally only affect “adventure” travelers
from the US. There are major efforts to control both snails and worms,
and a new drug called praziquantel is remarkably effective against the
worms, with a single dose controlling them for five years. A wonderful
topic for the Animal Planet show “Monsters Inside Us”. Flukes do occur
in the USA, and cause “swimmer’s itch”, but it is a different genus of
worms and not nearly as serious as schistosomiasis.
4. Planaria reproduce sexually, but are also capable of asexual
reproduction by literally splitting in two from front to back, and then
regenerating both symmetrical sides. This remarkable capacity for
regeneration is being exploited in research labs where scientists like
Phillip Newmark in our Cell and Developmental Biology department are
studying the genes and proteins involved (handout). We would like to
understand why we humans are only capable of regenerating certain
tissues and organs. The answer almost certainly involves at least in part
the remarkable properties of stem cells, which are capable of
regenerating both themselves and other more specialized cells.
Demonstrate by cutting flatworms in half, but one can actually cut them
in tens of pieces and each is capable of regenerating into a whole worm.
This implies that every part of the flatworm body “knows” what and
where it should be, and that information can be used to regenerate the
neighboring missing cells and tissues. It turns out that this is achieved by
specialized “adult stem cells” called neoblasts that regenerate the
missing parts - would that we could do the same for all of our body.
Schistosoma lifecycle.
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6. The third class are the Cestoda or tapeworms. These are primarily gut
parasites of vertebrates with extremely modified and simplified
morphology. A barbed head attaches to the gut lining, and the rest of the
body is repeated reproductive segments, which exit with faeces.
7. Phylum Nemertea: These are long flat colorful ribbonworms.
Entirely marine and about 1000 species known. They prey on other
animals using a unique proboscis. In contrast to the flatworms they
have a complete gut with an anus at the other end. As before, the
mouth forms from the blastopore created at gastrulation, while the
anus forms from a secondary invagination near the posterior end.
© Copyright Glencoe-McGraw Hill, 2000
8. Phylum Nematoda: These are called roundworms because they have
a body cavity, so in cross-section they are round. There are >20,000
species, and these are a major under-appreciated fauna in most
environments. A body cavity forms between the ectoderm and endoderm
and contains tissues formed from mesoderm, including a circulatory
system. This pseudocoelom (so-called because it is not considered a true
coelom, which must be lined with mesoderm) is kept open by hydrostatic
pressure against the outer hard cuticle. Alternating layers of longitudinal
and circular muscles allow sinusoidal movement.
9. Filariasis is a parasitic disease
caused by nematodes of two
species, Brugia malayi and
Wucheria bancrofti. These tropical
diseases are transmitted by
mosquitoes. Like flatworms, it is
difficult to develop drugs against
these animal parasites, as they have
similar physiology, mediated by
similar genes and proteins, to us.
The worms block lymph nodes,
and if untreated lead to massive
swelling, especially of the legs,
called elephantiasis.
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10. Caenorhabditis elegans is a free-living, tiny, 1mm-long, soil
nematode that has become a model organism in molecular biology. Each
of its 959 cells is fully described and located, all cell divisions, including
cell death, are known, and all connections between all nerves are known.
All of its ~100 million base pair DNA have been sequenced (indeed, the
first animal genome completely deciphered in 1998) to uncover all
~20,000 genes in its genome encoding all the different proteins needed to
make an ecologically functioning animal, including ~1000
chemoreceptors that I study (5% of all its genes/proteins).
11. Phylum Rotifera: Rotifers don’t look like worms, but their level of
complexity is similar, including a pseudocoelom body cavity. They
attach to a substrate and filter bacteria, protists, and algae using beating
cilia. We are not sure where they fit in phylogenetically.
12. Phylum Tardigrada: The tardigrades (meaning “slow walkers”) are
also known as “water bears”. They are tiny animals less than 2 mm in
length, with perhaps 1000 species, which live primarily in moist places
like on mosses. Their most distinctive feature is that they are able to
survive complete desiccation (as well as various other extreme insults,
some even suggest outer space!). Just add water and they come back to
life. They do this by packing their cells with sugars that protect them.
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