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CHAPTER 33
LECTURE
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Noncoelomate Invertebrates
Chapter 33
Porifera
• Parazoa
– Animals lacking tissues (and therefore organs) and a
definite symmetry
• 7000 marine species; 150 freshwater species
• Among the most abundant animals in the deep
ocean
Bryozoa (Ectoprocta)
Brachiopoda
Platyhelminthes
Cycliophora
Rotifera
Micrognathozoa
Acoela
Ctenophora
Cnidaria
Porifera
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• Most members lack
symmetry
• Various growth
forms
– Larval sponges
free-swimming
– Adults remain
attached – sessile
• Cell types
– Truly multicellular
– 3 functional layers
in “vase”
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• 3 layers
1. Outer epithelium
• Water comes in ostia, exits osculum
2. Mesohyl
• Middle layer – gelatinous matrix
• Spicules – needles of calcium carbonate
• Spongin – reinforcing tough protein fibers
3. Choanocytes
•
•
•
•
Collar cells
Flagellated – contributes to water circulation
Face internal cavity
Engulf and digest food from passing water
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• Sponge reproduction
– Asexual
• Fragmentation
– Sexual
• Choanocytes transform into sperm
• Sperm captured and passed to egg cell in mesohyl
• Development may occur within mother or in open
water
• Larva is planktonic; will settle and transform into
adult
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Eumetazoa
• Animals with distinct tissues
• Embryos have distinct layers
– Inner endoderm forms the gastrodermis
– Outer ectoderm forms the epidermis and nervous
system
– Middle mesoderm (only in bilateral animals) forms the
muscles
• True body symmetry
– Radial symmetry
– Bilateral symmetry
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Bryozoa (Ectoprocta)
Brachiopoda
Platyhelminthes
Cycliophora
Rotifera
Micrognathozoa
Acoela
Ctenophora
Cnidaria
Phylum Cnidaria
Porifera
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• Most marine, few fresh water species
• Diploblastic
• Bodies have distinct tissues but no organs
– No reproductive, circulatory, or excretory systems
• No concentrated nervous system
– Latticework of nerve cells
– Touch, gravity, light receptors
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• Cnidarians use nematocysts to capture prey
– Secreted within nematocyte
– Mechanism of discharge unknown
– Some carry venom
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• 2 basic body forms
– Polyps – cylindrical and sessile
– Medusa – umbrella-shaped and free-living
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• Body plan has single opening leading to
gastrovascular cavity
–
–
–
–
Site of digestion
Most gas exchange
Waste discharge
Formation of gametes in many
• 2 layers to body wall
1. Epidermis
2. Gastrodermis
– Mesoglea between layers
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• Gastrovascular space also serves as
hydrostatic skeleton
– Provides a rigid structure against which
muscles can operate
– Gives the animal shape
• Many polyp species build an exoskeleton
of chitin or calcium carbonate around
themselves
– Some build an internal skeleton
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• Cnidarian life cycle
– Some cnidarians occur only as polyps, and
others exist only as medusae, but many
alternate between these two phases
• Both phases consist of diploid individuals
– In general, in species having both polyp and
medusa in the life cycle, the medusa forms
gametes
•
•
•
•
•
Sexes separate
Gonochorism – individual is either male or female
Zygote develops into planktonic planula
Metamorphosis into polyp
Polyp produces medusae or other polyps asexually
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• Major evolutionary innovation in cnidarians
is extracellular digestion of food inside the
animal
– Digestion takes place partly in gastrovascular
cavity
– Cells then engulf fragments by phagocytosis
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4 or 5 classes
1. Anthozoa
– Sea anemones, most corals,
sea fans
– Solitary and colonial polyps
– Symbiotic dinoflagellates
(zooxanthellae)
photosynthesize and provide
nutrients to reef coral
– Coral reefs economically
important
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2. Cubozoa
– Box jellies
– Strong swimmers, voracious
fish predators
– Stings may be fatal to humans
3. Hydrozoa
– Hydroids, Hydra, Portuguese
man-of-war
– Both polyp and medusa
stages
– Only class with freshwater
members
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4. Scyphozoa
– Jellyfish
– Medusa more conspicuous and
complex
– Ring of muscle cells allows for
rhythmic contractions for
propulsion
5. Staurozoa
– Star jellies
– Resembles a medusa in most
ways but is attached to the
substratum by a sort of stalk that
emerges from the side opposite
the mouth
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Bryozoa (Ectoprocta)
Brachiopoda
Platyhelminthes
Cycliophora
Rotifera
Micrognathozoa
Acoela
Ctenophora
Cnidaria
Phylum
Ctenophora
Porifera
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• Known as comb jellies, sea
walnuts, or sea gooseberries
• 8 rows of comblike plates of
fused cilia that beat in a
coordinated fashion
• Many bioluminescent
• 2 tentacles covered with
colloblasts
– Discharge strong adhesive used
to capture prey
• Phylogenetic position unclear
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The Bilaterian Acoelomates
• Characterized by bilateral symmetry
• Allowed for high levels of specialization
• Bilaterians are traditionally classified by
the condition of their coelom
Bryozoa (Ectoprocta)
Brachiopoda
Platyhelminthes
Cycliophora
Rotifera
Micrognathozoa
Acoela
Ctenophora
Cnidaria
Porifera
– Acoelomates
– Pseudocoelomates
– Coelomates
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Phylum Platyhelminthes
• Flatworms are ciliated, soft-bodied animals
• Bodies are solid aside from an incomplete
digestive cavity
• Many species are parasitic
• Others are free-living
– Marine, freshwater, moist terrestrial
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• Only one opening to digestive cavity
– Muscular contractions in the pharynx allows
food to be ingested and torn into small bits
• Lack circulatory system
– Diffusion for gas transport
– Gut functions in digestion and food
distribution
– Some particles digested extracellularly
– Cells engulf particles by phagocytosis
– Tapeworms (parasitic flatworms) lack
digestive systems – absorb food directly
through body walls
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• Have an excretory and osmoregulatory
system
– Network of fine tubules runs through body
– Flame cells located on the side branches
• Flagella move water and excretory substances into
the tubules and then to pores located between the
epidermal cells through which the liquid is expelled
– Metabolic wastes are excreted into the gut
and eliminated through the mouth
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• Simple nervous system
– Anterior cerebral ganglion and nerve cords
– Eyespot can distinguish light from dark
• Reproduction
– Most are hermaphroditic
– Undergo sexual reproduction
– Also have capacity for asexual regeneration
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• 2 major groups
– Free-living Turbellaria
• Probably not monophyletic
• Dugesia – common planarian in bio labs
– Parasitic Neodermata
• Trematoda – flukes
– Attach within host body by suckers, anchors, or hooks
– Life cycle may have 2 or more hosts
– Clonorchis sinensis, oriental liver fluke
• Cercomeromorpha – tapeworms
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• One of most important trematodes to human
health are blood flukes Schistosoma
– Afflict 5% of world’s population
– About 800,000 people die each year from
schistosomiasis or bilharzia
– Fertilized egg must break through the wall of the
blood vessels in intestine or the urinary bladder to get
out
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• Cercomeromorpha – tapeworms
– Adult hangs onto inner wall of host intestine
using scolex
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• Most of tapeworm body is proglottids
– Complete hermaphroditic unit, containing both male and female
reproductive organs
– Formed continuously
• Beef tapeworm, Taenia saginata
– Frequent human parasite
– From eating uninspected rare beef
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Phylum Acoelomorpha
• Acoel flatworms were once considered basal
members of the phylum Platyhelminthes
• Have a primitive nervous system and lack a
digestive cavity
• Based on
molecular
evidence,
similarities are
convergent
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Phylum Cycliophora
• Discovery reported in
1995
• Striking circular mouth
surrounded by a ring of
cilia
• Anatomy and life cycle
are very unusual
• Live on the mouthparts of
claw lobsters on both
sides of the North Atlantic
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Pseudocoelomates
• Pseudocoelom – cavity that lies between tissues derived
from the mesoderm and those derived from the
endoderm
• Pseudocoelomic fluid performs the functions carried out
by a circulatory system in most coelomate animals
• Not monophyletic
Chordata
Echinodermata
Chaetognatha
Onychophora
Arthropoda
Tardigrada
Nematoda
Kinorhyncha
Loricifera
Nemertea
Mollusca
Annelida
Brachiopoda
Bryozoa (Ectoprocta)
Platyhelminthes
Cycliophora
Rotifera
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Phylum Nematoda
• Vinegar eels, eelworms, and other
roundworms
• Members of this phylum are found
everywhere – abundant and diverse
• Marine, freshwater, parasites, free-living
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• Bilaterally symmetrical and unsegmented
• Covered by a flexible, thick cuticle that is
molted as they grow
• Digestive system well developed
– Stylets – piercing organs near mouth
– Pharynx – creates sucking action
– Anus
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• Sexual reproduction
– Most gonochoric
– Sexual dimorphism – male smaller with
hooked end
– Internal fertilization
– Indirect development – egg, larva, adult
• Eutely
– Adults consist of a fixed number of cells
– Caenorhabditis elegans has only 959 cells
– Important in genetic and developmental
studies
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• Lifestyles
– Many are active hunters, preying on protists
and other small animals
– Others are parasites of plants
– Still others live within the bodies of larger
animals
• Largest known nematode, which can attain a
length of 9 m, parasitizes the placenta of sperm
whales
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• About 50 species cause human diseases
• Hookworms
– Common in southern U.S.
– Produce anemia
• Trichinella causes trichinosis
– Forms cysts in muscles
– Infection from eating undercooked meat
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• Pinworms, Enterobius vermicularis
– Infects 30% of children in U.S.
– Causes itching of the anus
• Ascaris lumbricoides – intestinal
roundworm
– Infects 1 in 6 worldwide
– Adult female can be 30 cm long
– Rare in areas with modern plumbing
• Serious tropical nematode diseases
– Filariasis
– Elephantiasis
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Phylum Rotifera
• Bilaterally symmetrical, unsegmented
pseudocoelomates
• Highly developed internal organs
• Corona – “wheel animals”
– Conspicuous ring of cilia at anterior end
– Used for locomotion and sweeping food into
the mouth
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