Download 38-2 What is an animal? - McGraw Hill Higher Education

Chapter 38: Simple animals: sponges
to flatworms
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What is an animal?
• Kingdom Animalia or Metazoa
• Multicellular
– division of labour with specialised cells
– in animals other than sponges, cells form tissues and
organs
• Embryonic development from a single-celled
zygote
• Chemoheterotrophs
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The origin of multicellularity
• Animals evolved from a protistan ancestor
• Theories of animal multicellularity
– ciliate protist in which partitioned cytoplasm gave rise to
an organism resembling flatworm
– flagellate protist similar to choanocytes of sponges
– two-layered organism similar to Trichoplax
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Fossil faunas
• Ediacaran fauna (600 million years ago)
– oldest animal fossils
 first recorded from Ediacara Hills in South Australia
– soft-bodied fauna preserved as impressions
– species resemble jellyfish and sea pens (Cnidaria) and
segmented worms (Annelida)
• Burgess Shale fauna (545–510 million years ago)
– large and diverse fauna of many phyla, especially
arthropods
– presence of exoskeletons
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Phylogeny of modern groups of
animals
• Subkingdom Parazoa (sponges)
• Subkingdom Eumetazoa (other animals)
• Eumetazoa divided into
– radially symmetrical animals (Cnidaria, Ctenophora)
– bilaterally symmetrical animals (Bilateria)
• Bilateria divided into
– protostomes (flatworms, segmented worms, molluscs,
arthropods)
– deuterostomes (echinoderms, chordates)
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Fig. 38.2: Phylogenetic tree of the
animal kingdom, Metazoa
Copyright © South Australian Museum, photograph N Pledge
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Question 1:
Which of the following is not a characteristic of
animals?
a) Multicellular
b) Membrane-bound organelles
c) Heterotrophic
d) Presence of a coelom
e) Zygote undergoes cleavage
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Protostomes and deuterostomes
• Two patterns of development based on the fate of
the blastopore and the formation of the coelom in
the embryo
• Protostomes
– blastopore becomes the mouth
– coelom forms by splitting of mesoderm (schizocoely)
– spiral cleavage, determinate development
• Deuterostomes
– blastopore becomes the anus
– coelom forms from outpocketing of gut (enterocoely), except in
vertebrates
– radial cleavage, indeterminate development
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Fig. 38.3: Protostome and
deuterostome
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Sponges (phylum Porifera)
• Sponges are aquatic animals with cell layers that
are not organised into tissues or organs
• Characteristics
–
–
–
–
choanocytes (collar cells) line internal cavity
pinacocytes make up outer covering (pinacoderm)
acellular mesohyl (protein matrix)
amoebocytes in mesohyl (amoebocytes normally
phagocytose food particles, but are totipotent, so they
can transform into other cell types)
– calcareous or siliceous spicules and/or spongin fibres
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Filter feeding
• Sponges filter food particles from water
– choanocytes create feeding currents by beating flagella
• Water enters through ostia and passes through
central atrium or spongocoel before being expelled
through the oscula
– in advanced sponges, water may pass through a series of
small chambers in mesohyl
• Pinacocytes phagocytose large particles
• Choanocytes trap small particles in collar
• Both transfer food to amoebocytes for digestion
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Fig. 38.6: Structure of a simple
sponge
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Polyps and medusae: phylum
Cnidaria
• Cnidarians are radially-symmetrical animals with
two tissue layers in the body wall
• Characteristics
–
–
–
–
–
diploblastic (endoderm, ectoderm)
nematocysts (stinging organelles)
sac-like body with gastrovascular cavity (coelenteron)
two body forms (medusa, polyp)
reproductive cycle usually passes through sexual
(medusa) and asexual (polyp) stages
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Fig. 38.7: Basic Cnidarian structure
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Nematocysts
• Nematocysts are produced in cnidocytes, usually
on tentacles or body wall
• Thread-like filament is coiled inside capsule
• When triggered, filament is everted and barbs
penetrate integument of prey or predator
• Some nematocysts inject proteinaceous toxin
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Fig. 38.8: Structure of a nematocyst
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Classification
•
•
•
•
Class Cubozoa (box jellyfish)
Class Scyphozoa (jellyfish)
Class Hydrozoa (hydroids and hydras)
Class Anthozoa (corals, sea anemones and allies)
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Class Cubozoa (box jellyfish)
• Cubozoans have a characteristic cube-shaped bell
• Other characteristics
– tentacles in groups of four on bell
– planulae released from the female medusa settle as
polyps
– adults move into shallow waters in November
– toxins delivered by batteries of nematocysts in the
tentacles may kill humans in minutes
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Class Scyphozoa
• Jellyfish, sea jellies
• Medusa with flattened or domed bell
– mouth on tubular manubrium
– stomach opens into four radial canals that connect with a
ring canal running around the margin of the bell
• Medusa stage is dominant
– planula settles as actinula
– actinula grows into scyphistoma that produces ephyrae
by horizontal splitting (strobilation)
– ephyrae develop into adult medusae
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Fig. 38.10: Life cycle of the jellyfish
Aurelia
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Class Hydrozoa
• Hydras, hydroids, siphonophores
• Polyp stage usually dominant
– solitary species (e.g. Hydra)
– colonial species (e.g. Obelia, Physalia)
• Colonial species with specialised polyps for
feeding and reproduction
• Siphonophores (e.g. Physalia) resemble jellyfish
– specialised polyps include the float, feeding tentacles and
male and female gonophores
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Fig. 38.14: Life cycle of the hydrozoan
Obelia
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Class Anthozoa
• Corals, soft corals, sea fans, sea pens, sea
anemones
• Medusa stage is suppressed
– polyps characteristically with an oral disc and
gastrovascular cavity partially divided by septa
(mesenteries)
– colonies formed by budding
• Hard corals produce a complex skeleton of CaCO3
– symbiotic zoochlorellae and zooxanthellae
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Fig. 38.17b: Structure of a sea
anemone
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Zoochlorellae and zooxanthellae
• Single-celled algae living intracellularly in many
cnidarians, most notably hard corals
• Symbiotic relationship
– coral provides algae with nutrients and a safe
environment
– algae provide coral with nitrogen and organic carbon
compounds
– removal of CO2 by algae assists with CaCO3 formation
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Question 2:
What colour would corals and anemones be that
had lost their photosynthetic algae zooxanthellae?
a) Brown
b) Green
c) White
d) Black
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Phylum Ctenophora
• Comb jellies resemble cnidarians but lack
nematocysts
– one species, Euchlora rubra, possesses nematocyst
• Characteristics
– fused cilia in longitudinal rows (comb rows) along body
– line of ciliated cells connects each comb row to an apical
organ, which includes statocyst
– digestive system with two anal pores
– feed by tapping prey with a pair of retractable, adhesive
tentacles
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Fig. 38.18: Ctenophores
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Phylum Platyhelminthes
• Free-living and parasitic flatworms
• Characteristics
–
–
–
–
–
–
–
triploblastic (ectoderm, mesoderm, endoderm)
acoelomate (no body cavity)
excretory protonephridia with flame cells
blind-ending digestive tract
cephalisation (anterior concentration of neurons)
ladder-like nervous system
dorso-ventrally flattened
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Fig. 38.19: Freshwater flatworm Dugelsa
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Classification
•
•
•
•
Class Turbellaria (free-living flatworms)
Class Monogenea (ectoparasitic flukes)
Class Trematoda (endoparasitic flukes)
Class Cestoda (tapeworms)
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Class Turbellaria
• Turbellaria are free-living predators or scavengers
– some turbellarians feed using a muscular pharynx on the
ventral surface
– temnocephalans live on crayfish and use their anterior
tentacles to catch prey
• Gas exchange takes place by diffusion across the
surface of the skin
• Characteristic rod-like rhabdites in the epidermis
are secreted by gland cells
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Class Trematoda
• Endoparasitic flukes have complicated life cycles
that involve two or more hosts
– larval stage(s) in intermediate host(s)
– adult stage in definitive host
• The trematode life cycle involves sexual and
asexual stages
– adults lay eggs that pass out in the faeces of the
definitive host
– miracidium hatches from egg, enters intermediate host
and becomes sporocyst
– each sporocyst gives rise to multiple rediae
– each redia gives rise to multiple cercariae that infect
definitive host
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Fig. 38.23: Life cycle of liver fluke
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Class Cestoda
• Tapeworms are highly-specialised parasites that live
in vertebrate guts as adults
– larvae usually pass through a vertebrate or invertebrate
intermediate host
• Attach to gut wall by scolex with hooks and suckers
– body composed of repeated units (proglottids) that form
behind the scolex and become detached at posterior end
– each proglottid contains male and female reproductive
organs
– cross-fertilisation occurs between two tapeworms or
between two proglottids on the same animal
– detached proglottids with fertilised eggs pass out in faeces
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Fig. 38.24: Human blood fluke
lifecycle
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Phylum Nemertea
• Ribbon worms catch prey with an eversible
proboscis
• Characteristics
–
–
–
–
–
triploblastic
acoelomate but with coelom-like rhynchocoel
excretory protonephridia with flame cells
one-way digestive tract with terminal anus
proboscis with stylet that may inject toxins into prey
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Summary
• Animals are multicellular, heterotrophic organisms
• Sponges (Parazoa) are the simplest animals in
terms of structure
• All other animals are Eumetazoan and
characterised by radial or bilateral symmetry
• Bilaterally symmetrical animals diverged along two
major lines of evolution: protostomes and
deuterostomes
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