BI113 Lecture 21

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BI-113 Lecture 21

• Diversity of Lower Invertebrate Phyla

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Phylum Platyhelminthes
triploblastic
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Another tissue layer

• Triploblastic animals with third germ layer
• Mesoderm
• Middle layer
• Forms muscles, skeletal elements and other organs
• The advent of three germ layers allows for the
  creation of an internal space
• Coelom

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Three germ cell layers
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Phylum Platyhelminthes

• the Flatworms
• 25,000 species
• Triploblastic - three cell layers
• Acoelomate - no coelom
• Bilateral symmetry
• Marine, freshwater, and terrestrial species
• Gut, when present, is a blind diverticulum
• No specialized gas exchange structures and no
  circulatory system
• All respiration and diffusion of gases takes
  place across tissues
• Radial cleavage of early embryo

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Reproduction in Platyhelminthes

• Many species are monoecious
• Simultaneous hermaphrodites
• Possess both sexual reproductive tissues in the
  same body
• Compared to dioecious
• In which the sexes are separate
• Distinct male and female morphology
• Transfer of gametes is by means of male
  intromittent structure or penis
• Sperm of flatworms are biflagellate
• 9 1 microtubule structure (unique in animals)

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Subdivisions of Platyhelminthes

• Class Turbellaria
• Class Cestoda
• Class Monogenea
• Class Trematoda

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Class Turbellaria

• Planarians
• Free-living, marine (most), freshwater and
  terrestrial forms
• Protrusable pharynx serves as mouth and anus
• Carnivores with extracellular digestion in
  gastrovascular cavity
• Great regenerative powers
• Many species with eyespots
• These serve as simple photoreceptors and have no
  image-forming capability
• May be 1 (usual), or many pair
• Anterior swelling/collection of nerve tissues
  forms a brain
• Osmoregulation by means of protonephridia (flame
  cells)

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Turbellarian morphology
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Turbellarian diversity
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Class Trematoda

• Flukes
• Internal parasites
• Some with serious health consequences for humans
• Often with very complicated life cycles
• Usually with a vertebrate host (often a fish, but
  also other vertebrates) and 2-3 intermediate
  hosts (often a mollusk) that serve as maturation
  sites for a variety of larval stages
• As endoparasites, many are facultative anaerobes
• Monoecious
• With many variations on the arrangement of male
  and female reproductive structures

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Liver fluke

• Opisthorchis sinensis - Chinese liver fluke
• Eggs eaten by snail (1st intermediate host)
• Several larval stages in snail miracidium,
  sporocyst and redia
• Cercaria stage leaves snail and burrows into
  flesh of fish (2nd intermediate host)
• Forms an encysted stage - the metacercaria
• These are eaten by humans that serve as the
  definitive host
• Adult stage resides in the liver sinusoids

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Adult flukes deposit fully developed eggs that
are passed in the feces (1). After ingestion by
a suitable snail (first intermediate host 2),
the eggs release miracidia (2a), which undergo in
the snail several developmental stages
(sporocysts2b, rediae2c, cercariae2d). Cercari
ae are released from the snail (3) and penetrate
freshwater fish (second intermediate host),
encysting as metacercariae in the muscles or
under the scales (4).
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The mammalian definitive host (cats, dogs, and
various fish-eating mammals including humans)
become infected by ingesting undercooked fish
containing metacercariae. After ingestion, the
metacercariae excyst in the duodenum (5) and
ascend through the ampulla of Vater into the
biliary ducts, where they attach and develop into
adults, which lay eggs after 3 to 4 weeks
(6). The adult flukes (O. viverrini 5 mm to 10
mm by 1 mm to 2 mm O. felineus 7 mm to 12 mm by
2 mm to 3 mm) reside in the biliary and
pancreatic ducts of the mammalian host, where
they attach to the mucosa
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Blood fluke

• Schistosoma mansoni - blood fluke
• Found in tropical Africa and Americas
• Dioecious - with separate sexes
• Live in intestinal blood vessels of humans
• Intermediate host is a snail

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Life cycle of Schistosoma
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Class Cestoda

• Tapeworms
• Internal gut parasites
• No mouth or digestive tract
• Nutrient absorption occurs across integument (in
  this case, a syncytial layer)
• Anterior structure - scolex
• Modified for attachment to the lining of the gut,
  often with multiple hooks and suckers
• Body is comprised of many segments - proglottids
• These are largely comprised of reproductive
  tissues
• Monoecious - complete reproductive tracts (male
  and female) occur in each proglottid

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Cestode characteristics

• Definitive host is always a vertebrate
• With one or more intermediate hosts (usually a
  vertebrate or an arthropod)
• Taenia solium - human tapeworm
• Can reach length of 7 metres (23 ft)

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Taenia life cycle
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Pseudocoelomates
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Aschelminthes

• A heterogeneous group of pseudocoelomate animal
  phyla
• Many are worm-like in their body morphology
• Most are marine
• Many are sediment dwellers, others are parasitic
• Phyla include
• Kinorhyncha (100 - marine, sediment)
• Gastrotricha (300 - marine/freshwater sediment)
• Gnathostomulida (80 - marine, sediment)
• Acanthocephala (500 - parasitic)
• Priapulida (10 - marine, sediment)

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Aschelminth phyla
Priapulida
Kinorhyncha
Gastrotricha
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Phylum Rotifera

• the Rotifers
• 1,800 species
• Complete digestive tract
• Life history
• Parthenogenesis
• Populations of diploid females
• Amictic eggs are diploid and capable of
  development into an adult female without
  fertilization
• During unfavorable environmental conditions, eggs
  are haploid, some of these develop into males and
  some into females
• These females produce mictic eggs which require
  fertilization
• The diploid zygotes which result become resting
  eggs which can survive harsh environment until
  conditions are favorable once again
• At that time the resting eggs hatch and develop
  into females which reproduce parthenogenetically
  once again

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Phylum Nematoda

• The Roundworms
• 20,000 species
• Pseudocoelomate
• With internal body cavity formed between the
  endoderm and the mesoderm (being comprised
  largely of longitudinal muscle)
• Bilateral symmetry
• Body integrity maintained by hydrostatic pressure
• Live in marine, fresh water, and terrestrial
  habitats
• Most are dioecious

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Different ways of life

• Free-living and parasitic forms
• Many free-living species are important in the
  soil community
• Parasites include the worms responsible for
  trichinosis (Trichinella spiralis), become
  encysted within muscle tissue
• Some species are very small (microscopic)
• Others are large
• The largest nematode is a parasite within the
  uterus (placentae) of the sperm whale
• Reaches lengths of 9 metres - the worm, not the
  whale

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Nematode anatomy
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Nematode diversity
Trichanella spiralis encysted in muscle
Parascaris sp. in small intestine
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Caenorhabditis elegans
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• Adult worms (1) live in the lumen of the small
  intestine
• A female may produce up to 240,000 eggs per day,
  which are passed with the feces (2)
• Fertile eggs embryonate and become infective
  after 18 days to several weeks (3), depending on
  the environmental conditions (optimum moist,
  warm, shaded soil)
• After infective eggs are swallowed (4), the
  larvae hatch (5), invade the intestinal mucosa,
  and are carried via the portal, then systemic
  circulation to the lungs (6)

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• The larvae mature further in the lungs (10 to 14
  days), penetrate the alveolar walls, ascend the
  bronchial tree to the throat, and are swallowed
  (7)
• Upon reaching the small intestine, they develop
  into adult worms (1)
• Between 2 and 3 months are required from
  ingestion of the infective eggs to oviposition by
  the adult female
• Adult worms can live 1 to 2 years