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Noncoelomate Invertebrates
Chapter 33
Invertebrate Phylogeny Revolution
Scientists generally agree about the
taxonomic classification of 36 animal phyla
-They do disagree, however, about how
these are interrelated
Traditional reconstructions lump together
phyla that share major features of body plan
New reconstructions employ molecular
comparisons of rRNA and other genes
2
Invertebrate Phylogeny Revolution
Both old and new phylogenies agree that:
-Porifera first separated from Eumetazoa
-Among the eumetazoans, Cnidaria and
Ctenophora branch out before Bilateria
-Bilateria are divided into two groups:
protostomes and deuterostomes
Modern phylogeny differs radically from
traditional phylogeny in construction of the
protostome lineage
3
Invertebrate Phylogeny Revolution
Traditionally, the branches of animals are
based on the nature of the body cavity:
-Acoelomates = Have no body cavity
-Pseudocoelomates = Have pseudocoel
-Coelomates = Have coelom
Acoelomates and pseudocoelomates are
always protostomes
Coelomates are either protostomes or
deuterostomes
4
Invertebrate Phylogeny Revolution
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Invertebrate Phylogeny Revolution
Modern protostome phylogeny distinguishes
between two major clades that have evolved
independently:
-Spiralians
-Ecdysozoans
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Invertebrate Phylogeny Revolution
Spiralians
-Grow by adding mass to an existing body
-Two main groups
-Lophotrochozoa: mostly coelomates
-Include the phyla Brachiopoda,
Mollusca and Annelida
-Platyzoa: mostly acoelomates
-Include the phyla Platyhelminthes,
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and Micrognathozoa
Invertebrate Phylogeny Revolution
Ecdysozoans
-Increase in size by molting their external
skeletons
-Two phyla have been particularly successful
-Nematoda: pseudocoelomates
-Roundworms
-Arthropoda: coelomates
-Insects, crustaceans, and others
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Invertebrate Phylogeny Revolution
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Parazoa
Parazoans lack tissues, organs and a definite
symmetry
-However, they have complex multicellularity
Sponges, phylum Porifera, are parazoans
-Include marine and freshwater species
-Larval sponges are free-swimming, but
adults are anchored onto submerged objects
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Parazoa
Adult sponges are shaped like a vase
Walls of the “vase”
have three
functional layers
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Parazoa
1. Inner layer = Specialized flagellate cells
called choanocytes, or collar cells
2. Central layer = Gelatinous, protein-rich
matrix called the mesohyl
3. Outer layer = Protective epithelium
Mesohyl may contain spicules and/or fibers
of a tough protein called spongin
-These strengthen the body of the sponge
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Parazoa
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Parazoa
Sponges are “filter-feeders”
-Beating of the choanocyte flagella draws
water through numerous small pores
-Brings in food and oxygen and expels
wastes
-Small organisms are filtered from the
water, which flows through passageways
-Eventually exits from the osculum
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Parazoa
Sponge reproduction
-Asexual = Fragmentation
-Sexual = Egg and sperm
-Larval sponges use cilia for swimming
-Settle down on a substrate
-Transform into adults
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Eumetazoa
Eumetazoans are 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
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Eumetazoa
Eumetazoans also evolved true body symmetry
-Radiata = Exhibit radial symmetry
-Phylum Cnidaria
-Phylum = Ctenophora
-Bilateria = Exhibit radial symmetry
-All other animals
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Phylum Cnidaria
Cnidarians are nearly all marine
-Bodies have distinct tissues but no organs
Cnidarians are diploblastic
-Ectoderm (epidermis) and endoderm
(gastrodermis)
-In between, a layer of gelatinous material
(mesoglea)
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Phylum Cnidaria
Cnidarians are carnivores
They have two basic body forms
-Polyps = Cylindrical and sessile
-Medusae = Umbrella-shaped and free-living
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Phylum Cnidaria
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Phylum Cnidaria
Some cnidarians exist as either medusae or
polyps, while others alternate between the
two phases
Polyps may reproduce sexually or asexually
-Asexual reproduction occurs by budding and
may produce a new polyp or medusae
Medusae reproduce sexually
-Form free-swimming larvae called planulae
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Phylum Cnidaria
Cnidarians have internal extracellular digestion
-Digestion begins with extracellular
fragmentation in the gastrovascular cavity
-This is followed by phagocytosis and
intracellular digestion
Cnidarians have no circulatory, respiratory or
excretory systems
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Phylum Cnidaria
Cnidarians have unique specialized cells on
their epidermis called cnidocytes
The nematocyst is a special type of cnidocyte
-Contains a small but powerful harpoon
-Used for food acquisition and defense
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Phylum Cnidaria
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Four Classes of Cnidarians
Hydrozoa (hydroids)
Scyphozoa (jellyfish)
Cubozoa (box jellyfish) Anthozoa (corals
and sea anemones)
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Phylum Ctenophora
A small phylum whose members are known as
comb jellies, sea walnuts or sea gooseberries
Structurally more complex
than cnidarians
Capture prey with a
colloblast
-Contains strong adhesive
material
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Phylum Ctenophora
Ctenophores propel themselves through water
with eight rows of fused cilia
They are triploblastic
-Have true muscles cells derived from the
mesoderm
Their mode of symmetry is not strictly radial as
in cnidarians
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The Bilaterian Acoelomates
The Bilateria are characterized by bilateral
symmetry
-Allowed for high levels of specialization
Bilaterians are traditionally classified by the
condition of their coelom
-Acoelomates
-Pseudoceolomates
-Coelomates
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Phylum Platyhelminthes
The flatworms are soft-bodied animals
-Many species are parasitic
-Others are free-living
Move by ciliated
epithelial cells
Have developed
musculature
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Phylum Platyhelminthes
Flatworms have an incomplete digestive cavity
with only one opening
-Therefore, they cannot feed continuously
Muscular contractions in the pharynx allows
food to be ingested and torn into small bits
Tapeworms (parasitic flatworms) lack digestive
systems
-Absorb food directly through body walls 31
Phylum Platyhelminthes
Flatworms have an excretory and
osmoregulatory system
-Network of fine tubules runs through body
-Flame cells located on the side branches
-Primary function = water balance
-Secondary function = excretion
Metabolic wastes are excreted into the gut
and eliminated through the mouth
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Phylum Platyhelminthes
Flatworms lack a circulatory system
-Flattened body shape and highly-branched
gut utilize diffusion
They have a simple nervous system
-Eyespot can distinguish light from dark
Most are hermaphroditic
-Undergo sexual reproduction
-Also have capacity for asexual regeneration
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Phylum Platyhelminthes
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Phylum Platyhelminthes
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Phylum Platyhelminthes
Flatworms have been traditionally grouped into
four major classes
-Turbellaria Free-living
-Monogenea
-Trematoda
Parasitic
-Cestoda
New studies group all three parasitic classes
into a single group, Neodermata
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Phylum Platyhelminthes
Class Turbellaria
-Free-living flatworms
-Recently shown to be polyphyletic
-Found in freshwater, marine and even
terrestrial environments
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Phylum Platyhelminthes
Classes Monogenea and Trematoda
-Flukes
-Live as ectoparasites or endo-parasites in
the bodies of other animals
-Attach by suckers, anchors or hooks
-Most have a life cycle that involves two or
more hosts
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Phylum Platyhelminthes
Clonorchis sinensis: Oriental liver fluke
-Complex life cycle
-Miracidium – Ciliated stage in egg
-Rediae – Nonciliated larvae produced
within sporocyst
-Cercaria – Tadpole-like larval stage
-Metacercaria – Adult stage produced
within cysts
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Phylum Platyhelminthes
Other important flukes are the blood flukes of
the genus Schistosoma
-Three species cause the disease
schistosomiasis, or bilharzia
-Worms coat themselves with host’s own
antigens
-Thus, are immunologically invisible
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Phylum Platyhelminthes
Class Cestoda
-Tapeworms
-Live as parasites within the bodies of other
animals
-Most species occur
in the intestines of
vertebrates
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Phylum Platyhelminthes
Class Cestoda
-Long flat bodies divided into three zones
-Scolex = Attachment organ
-Neck = Unsegmented portion
-Proglottids = Repetitive sections
-Each a complex hermaphroditic unit
-Taenia saginata is the beef tapeworm
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Phylum Platyhelminthes
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Phylum Platyhelminthes
Acoel flatworms were once considered basal
members of the phylum Platyhelminthes
-Have a primitive nervous system and lack a
digestive cavity
Are now classified
in their own phyla,
Acoela
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Phylum Nemertea
Nemerteans are often called ribbon worms or
proboscis worms
Are long animals
that can stretch
to several meters
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Phylum Nemertea
Nemerteans possess a fluid-filled sac, the
rhynchocoel: a true coelomic cavity
Are the simplest animals with a complete
digestive system (mouth and anus)
Have a closed circulatory system
They are not related to flatworms
-Are now classified with other coelomate
protostomes in the Lophotrochozoa
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Phylum Cycliophora
Discovered in 1995
Acoelomates that live on
the mouthparts of claw
lobsters
Have unique anatomy
and form of sexual
reproduction
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The Pseudocoelomates
Possess a pseudocoel, which is a cavity
between the mesoderm and endoderm
-Serves as a hydrostatic skeleton, against
which the animal’s muscles can work
Pseudocoelomates lack a defined circulatory
system
-Role is performed by fluids that move
within the pseudocoel
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Phylum Nematoda
Nematodes are roundworms comprising
many species
-Found in marine, freshwater and soil
habitats
Are bilaterally symmetrical and unsegmented
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Phylum Nematoda
Nematodes are covered with a flexible, thick
cuticle
Are ecdysozoans
-Molt cuticle four times
Lack specialized respiratory organs and
exchange oxygen through their cuticles
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Phylum Nematoda
Mouth is equipped with piercing organs called
stylets
Food passes through the mouth by the
sucking action of the pharynx
Undigested material is eliminated through the
anus
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Phylum Nematoda
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Phylum Nematoda
Nematodes usually reproduce sexually
-Sexes are separate
Adults of some species exhibit eutely
-Have a fixed number of cells
-Caenorhabditis elegans has only 959 cells
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Phylum Nematoda
Many nematodes are active hunters, preying
on protists and other small animals
Others are parasites of plants
Still others live within the bodies of larger
animals
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Phylum Nematoda
Nematodes can cause diseases in humans
-Trichinella causes
trichinosis
-Forms cysts in
muscles
-Ascaris lumbricoides – Intestinal roundworm
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Phylum Rotifera
Rotifers are bilaterally symmetrical
unsegmented pseudocoelomates
-They have complex internal organs
They propel themselves through water by
rapidly beating thick cilia at their heads
-“Wheel animals”
They gather food via a conspicuous organ
called the corona
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Phylum Rotifera
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