Lophotrochozoa

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Lophotrochozoa

• 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.

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Mesozoa

• 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.

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 tolweb.org/Mesozoa/2484
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Mesozoan characteristics

• 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. 

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Mesozoa

• 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.

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Phylum Nemertea (Rhynchocoela) Ribbonworms

• 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.

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8.19
Baseodiscus mexicanus a nemertean from the
Galapagos Islands
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Phylum Nemertea (Rhynchocoela) Ribbonworms

• 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.

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http//www.dnr.sc.gov/marine/sertc/images/photo20
gallery/nemertean.jpg
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Phylum Nemertea (Rhynchocoela) Ribbonworms

• 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.

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8.18
Internal structure of female ribbon
worm (above). Nemertean with proboscis extended
(right)
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Clade Gnathifera

• 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,
  free-living aquatic animals and the other phylum
  (Acanthocephala) are wormlike endoparasites.

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Phylum Gnathostomulida

• 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.

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Phylum Gnathostomulida

• 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.

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8.20
Gnathostomula jenneri
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Micrognathozoa

• 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.

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Phylum Rotifera

• 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.

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9.1
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Phylum Rotifera

• 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.

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Phylum Rotifera

• 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.

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http//www.arcodiv.org/seaice/rotifers/Antarctic_r
otifer_Philodina_gregaria_400x300.jpg
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Phylum Rotifera

• Because of the problem of accumulation of
  deleterious mutations in lineages of exclusively
  asexually reproducing animals (a process known as
  Mullers 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.

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Phylum Acanthocephala

• 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.

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http//rydberg.biology.colostate.edu/Dissections/a
canthocephala/acanthfemmal.jpg
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9.3
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http//upload.wikimedia.org/wikipedia/commons/9/99
/Acanthocephala_Rhadinorhynchus.jpg
Acanthocephalan proboscis
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Phylum Acanthocephala

• 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 hosts
  gut.
• As is true in cestodes, Acanthocephalans lack a
  gut.

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Phylum Acanthocephala

• 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.

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Phylum Acanthocephala

• 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.

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Phylum Acanthocephala

• The change in behavior appears to be caused by
  the Acanthocephalan pumping a serotonin-boosting
  molecule into the Gammarus brain.
• This causes the Gammarus to think its having sex
  and cling as it would if mating. Interestingly,
  the parasites manipulation also causes female
  Gammarus to mimic the males mating behavior.

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Phylum Acanthocephala

• 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 parasites 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.