Transcript Lophotrochozoa

Lophotrochozoa
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Although the relationships between
Lophotrochozoan phyla are not well resolved a
set of 6 somewhat obscure phyla appear to be
most closely related to the Platyhelminthes.
These are the Mesozoa, Nemertea and four
phyla grouped together in a clade called the
Gnathifera.
Mesozoa
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The Mesozoa are tiny ciliated animals
ranging in size from only 0.5 mm to 7mm
in length.
Mesozoans are very specialized parasites
(or in some cases symbionts) of marine
invertebrates. About 50 species known.
tolweb.org/Mesozoa/2484
Mesozoan characteristics
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Bilaterally symmetrical.
No organs or tissues.
Body contains no internal cavity.
Possesses no gut.
Body only two cell layers in most places.
No nervous system.
Mesozoa
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Mesozoans are made up of only 20-30 cells arranged in
two layers, which are not homologous to the germ layers
of other protostomes.
They do not undergo gastrulation unlike other diploblasts
and triploblasts.
Despite their simplicity molecular evidence suggests that
Mesozoans are derived from triploblastic organisms.
Phylum Nemertea (Rhynchocoela)
Ribbonworms
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The nemerteans (ribbon worms) are long,
marine, predatory worms and there are
about 1000 species known.
Most are less than 20cm in length, but
others are many meters in length.
Figure 14.25
8.19
Baseodiscus mexicanus a nemertean from
the Galapagos Islands
Phylum Nemertea (Rhynchocoela)
Ribbonworms
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The general body plan of nemerteans is similar to that of
turbellarians. Like turbellarians they have a ciliated
epidermis and possess a large number of gland cells.
They also have flame cells.
Unlike members of the Platyhelminthes nemerteans have
a complete gut with a mouth and anus and a true
circulatory system. The flame cells also are associated
with the circulatory system and so are used to eliminate
metabolic wastes (excretion) rather than osmoregulation
as in Platyhelminthes.
http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/nemertean.jpg
Phylum Nemertea (Rhynchocoela)
Ribbonworms
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Prey is captured using a long muscular proboscis armed
with a barb called a stylet.
The proboscis lies in a body cavity called the rhynchocoel
and muscular pressure on fluid in the rhynchocoel causes
the proboscis to be quickly everted. The rhynchocoel
appears to be a modified coelom.
The prey is wrapped in the sticky, slime-covered, proboscis
and stabbed repeatedly with the stylet. Neurotoxins in the
slime incapacitate the prey.
Figure 14.24a
Figure 14.24b
8.18
Internal structure of female ribbon worm
(above).
Nemertean with proboscis extended (right)
Clade Gnathifera
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The clade Gnathifera (“jaw bearing”) groups four
phyla together on the basis that they all possess
small very similarly structured jaws.
Members of three phyla (Gnathostomulida,
Micrognathozoa, and Rotifera) are tiny, freeliving aquatic animals and the other phylum
(Acanthocephala) are wormlike endoparasites.
Phylum Gnathostomulida
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The first Gnathostomulid was not discovered until
1928 and only about 80 species are known.
They are tiny (0.5-1mm long) wormlike animals
that live in the interstitial spaces of sand and silt.
They scrape bacteria and algae from the
substratum using a pair of jaws in the pharynx,
which is similar in structure to the muscular
pharynx (mastax) of rotifers.
Phylum Gnathostomulida
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Because they lack a circulatory system,
and anus gnathostomulids were first
classed as turbellarians.
More recently it has been suggested that
they are more closely related to the phyla
Rotifera and Acanthocephala.
Figure 14.27
8.20
Gnathostomula jenneri
Micrognathozoa
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The first and only species known was collected
in 1994. Like gnathostomulids, they are tiny
(142uM) interstitial inhabitants and consume
bacteria, blue-green algae and similar tiny food
items.
They have a two-part head, thorax and
abdomen and a very complex jaw system made
up of multiple plates and teeth.
Phylum Rotifera
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Rotifers are named for their characteristic
ciliated crown or corona, which when it beats
looks like a rotating wheel.
Rotifers are tiny animals (most are 100-300µm
long and the largest only 3mm long) the
majority of which live in freshwater and are
benthic inhabitants (live on the bottom).
About 2000 species have been described.
Figure 15.18
9.1
Phylum Rotifera
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The beating of the cilia in the corona draws in
plankton-containing water for food.
The mouth opens to a modified muscular
pharynx known as a mastax, which is a
structure unique to rotifers.
The mastax has a set of complex jaws, which
are used to grasp and chew food.
Phylum Rotifera
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One group of rotifers, the Bdelloid rotifers,
are very unusual in that there are no
males, hermaphrodites, or evidence of
meiosis.
Molecular evidence suggests that there
has been only asexual reproduction in this
group for several million years.
http://www.arcodiv.org/seaice/rotifers/Antarctic_rotifer_Philodina_gregaria_400x300.jpg
Phylum Rotifera
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Because of the problem of accumulation of
deleterious mutations in lineages of exclusively
asexually reproducing animals (a process known
as Muller’s ratchet) it is unclear how the
bdelloids have been able to dispense with sexual
reproduction entirely.
Other rotifers reproduce using a combination of
sexual and asexual reproduction.
Phylum Acanthocephala
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Acanthocephalans are commonly known
as spiny-headed worms because of the
spiny eversible proboscis they use to
attach to the gut of their host.
All 1100 species of Acanthocephalan are
endoparasitic and most parasitize fish,
birds and mammals.
http://rydberg.biology.colostate.edu/Dissections/acanthocephala/acanthfemmal.jpg
Figure 15.20
9.3
http://upload.wikimedia.org/wikipedia/commons/9/99/Acanthocephala_Rhadinorhynchu
s.jpg
Acanthocephalan proboscis
Phylum Acanthocephala
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The body wall is covered with numerous
minute depressions which enormously
increase the surface are of the tegument
and facilitates (as in cestodes) the
absorption of food from the host’s gut.
As is true in cestodes, Acanthocephalans
lack a gut.
Phylum Acanthocephala
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Acanthocephalans have a lifecycle in
which a vertebrate is the definitive host
and an invertebrate the intermediate host.
Acanthocephalans, as other parasites do,
modify the behavior of the intermediate
host to enhance the chances of its being
eaten.
Phylum Acanthocephala
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For example, acanthocephalans that parasitize
Gammarus, a small freshwater crustacean,
cause the Gammarus to alter its behavior in the
presence of ducks, a common predator.
Instead of diving to the bottom when a duck
appears, the Gammarus swims into the light and
grasps tightly onto a piece of vegetation, greatly
increasing its chances of being eaten.
Phylum Acanthocephala
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The change in behavior appears to be caused by
the Acanthocephalan pumping a serotoninboosting molecule into the Gammarus’ brain.
This causes the Gammarus to think it’s having
sex and cling as it would if mating. Interestingly,
the parasite’s manipulation also causes female
Gammarus to mimic the males mating behavior.
Phylum Acanthocephala
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Another Acanthocephalan that parasitizes pill
bugs causes them to reverse their normal
behavior and avoid humid, dark areas.
Instead they wander in the open where they are
much more vulnerable to birds, the
acanthocephalans definitive host.
The parasite’s manipulations are very effective.
Although fewer than 1% of pill bugs are typically
infected with acanthocephalan parasites 30% of
pill bugs delivered to nestlings are infected.