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Position in Animal Kingdom
Chapter 13
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Radiate
Animals
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Both phyla Cnidaria and Ctenophora make up
the radiate animals.
Biradial symmetry is radial symmetry limited
to two planes that create mirror images.
Other eumetazoans have bilateral symmetry
or their radial symmetry is derived from a
bilateral ancestor.
Neither Cnidaria nor Ctenophora have
advanced beyond tissue level of organization
although a few organs are seen.
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Biological Contributions
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Both phyla have two well-defined germ
layers: ectoderm and endoderm;
mesoderm may be derived from ectoderm.
There is an internal body cavity: the
gastrovascular cavity.
Extracellular digestion occurs in the
gastrovascular cavity; gastrodermal cells
accomplish cellular digestion.
Definition
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The Cnidaria are tentacle-bearing
Metazoa, of the tissue grade of
construction, with primary radial
symmetry, composed essentially of
two epithelia with a gelatinous matrix
(mesoglea) between them, with
nematocysts and with only one internal
cavity, the digestive cavity, with a
mouth, but no anus.
Biological Contributions
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Most have tentacles, which are extensible
projections for food capture.
Radiates are the simplest animals with nerve
cells; there is no central nervous system.
Radiates are the simplest animals with sense
organs: statocysts and ocelli.
Locomotion is by muscular contraction or ciliary
comb plates.
The polyp and medusa forms allow wider
ecological possibilities.
Characteristics
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Entirely aquatic, some in fresh water,
but mostly marine.
Primary radial symmetry around a
longitudinal axis with oral and aboral
ends; No definite head
Characteristics
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Characteristics
Two basic
types of
individuals:
polyps and
medusae.
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Special cells, cnidocytes (from which the
phylum gets its name), with stinging
organelles, nematocysts, in either
epidermis, gastrodermis or both.
A primitive nervous system consisting of a
nerve net at the base of the epidermal
and gastrodermal layers, with impulse
transmission tending to be radiating and with
typically nonpolarized synaptic junctions.
Figure 13-2
Characteristics
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Most feed on zooplankton, although
some utilize larger animals and some
are suspension feeders on fine
particulate matter.
Prey is caught, immobilized and killed
with the nematocysts on the tentacles
and digestion is initially extracellular,
then intracellular.
Form and Function
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Characteristics
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Asexual reproduction by budding (in polyps) or
sexual reproduction by gametes (in all medusae
and some polyps).
Sexual forms monoecious or dioecious;
Primitive gonads; Ciliated free-swimming
planula larva in life cycle of most. (Image is from
the Biodidac image data base maintained at the university
of Ottawa.)
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No respiratory or excretory system and no
coelomic cavity.
Basic Body Forms
Polymorphism (poly = many; Morph
= form). Many members of phylum
display dimorphism and polymorphism.
Two major morphological types are
polyp and medusa, but there can be
many different types of polyp in a
colony creating polymorphic colonies.
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Figure 13-1 B
Polyp = hydroid form,
adapted for sedentary
or sessile life. Tubular
body w/ mouth at one
end surrounded by
tentacles. Aboral end
usually attached to a
substratum by a pedal
disc or by skeletal
secretion or by stolons
(root-like outgrowths).
Colonial Hydroids (Class
Hydrozoa)
Basic Body Forms
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May live singly or in
colonies. Colonies of
some species w/ more
than one kind of
individual (zooid or
"person"), each
specialized for a certain
function, e.G. Feeding,
reproduction or defense.
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Usually have life cycles
featuring both polyp and
medusa stages - can take
advantage of feeding in a
bottom environment as a
polyp and in the open
water as a medusa.
Complex Hydrozoan Colonies
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Basic Body Forms
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Medusa = jellyfish form,
free swimming and sexually
mature form,
„ Bell-shaped or umbrellashaped bodies,
„ tetramerous symmetry,
„ Mouth centered on
concave side, tentacles
and one or more types of
sense organs borne on
rim of umbrella.
The Portuguese Man-owar is actually a
polymorphic colony of
individual zooids
serving differing
functions.
Basic Body Forms
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Physalia from Jamaica
Medusa provides dispersal mechanism
offspring don’t compete with parents.
Mesoglea much thicker than in polyp constitutes bulk of animal - makes it
buoyant = "jelly" of jellyfish.
Nematocysts
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Figure 13-3
Functions:
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Hold prey.
Provide adhesive which aids in
locomotion by attaching
tentacles to substrate.
Penetrate and anchor in prey
and poison it.
cnidocil
Nematocysts
Nematocyst Poison
• Chemical and
mechanical stimuli
• Discharge 2 m/sec
• Penetrates most
biological substances
• Tips open, closed,
some wrap
• Contents injected,
adhesives, toxins
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Most dangerous to man is Cubozoan
jellyfish, the sea wasp (Chironex
fleckeri). Caused more human
suffering and death off Australian
coasts than Physalia (Portuguese manof-war) has in any of its home waters.
Symptoms range from burning pain at
site of contact with tentacles through
skin lesions and eruptions of various
sorts, often severe enough to leave
scars, to generate great pain, fever,
prostration, and respiratory
interference. Most severe rxns due to
shock = allergic rxn. "Sting" of most is
imperceptible to humans. Problems
usually occur only when repeatedly
stung.
the sea wasp, Chironex fleckeri a box jellyfish (Cubozoa)…
Nervous System
…and one of the deadliest
creatures known!
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a box jelly baby
http://www.ucmp.berkeley.edu/cnidaria/Chironex.html
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First animals to possess one,
so of much research interest.
Diffuse system = nerve net.
Conduction radiates. Synaptic
vesicles on both sides of
many synapses, so bidirectional.
Sense Organs / Statocyst
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Balance organ.
Ball of sandy or stony
material shifts when
animal moves.
It stimulates surrounding
neurons and provides
animal with information
about its position.
caspar.bgsu.edu/.../Neuroet
hology/
labs/images/Statocyst.jpg.
Sense Organs /Ocelli and
Rhopalium
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Significant that first organs are for
perception of external world.
Ocellus is first primitive light-sensing
organ.
Usually found on bell of scyphozoan
and Cubozoan medusae.
Often combined with statocyst into
rhopalium.
Class Hydrozoa
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Class Hydrozoa
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Colonies with polymorphism very
common (e.G. Hydractinia) and
Physalia. What is adaptive value of
colonies?
If can’t specialize body areas into
organs (no true mesoderm), specialize
individuals in a colony. Get
“superorganism”.
700 species, majority marine and colonial, a
few fresh water (F.W.) Species (the only F.W.
Cnidarians).
Typical life cycle includes both asexual polyp
and sexual medusa (some with no polyp
and some with no medusa). Includes hydra
which is not typical, but much studied because
easy to get and maintain.
Class Hydrozoa – Life Cycles
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There is general trend to reduced medusa
stage and increased colonialism and
polymorphism.
In the previous scheme, the medusa is the
most primitive form because it is sexual and
therefore represents an adult form.
Polyp is larval stage. Budding produced more
polyps asexually. The budding is equivalent
to formation of medusa buds in present
polyps.
Scyphozoa, Cubozoa and
Anthozoa
Class Hydrozoa – Life Cycles
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With the evolution of polyp, some
groups reduced the medusa. First it
remained attached, then the polyp
epidermis itself produced the gametes.
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Class Scyphozoa (the Common
Jellyfishes)
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Jellyfishes (classes Scyphozoa and
Cubozoa) are predominantly medusae
w/ polyp stage very reduced or
completely absent.
Corals and anemones (class Anthozoa)
are entirely polyps, no medusa.
Class Scyphozoa
Pelagic cnidarians in which the medusa is the
dominant and conspicuous form.
Often seen in coastal waters and usually
feared by local swimmers.
Only about 200 species, but very abundant,
so play important ecological role.
Found everywhere in oceans, extending to
depths of 3000 m or more.
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Larger and more brightly colored than
hydromedusae. Compensate for
increased size by increasing complexity
of gastrovascular system (oral arms
and radial canals) and by better control
of swimming (no velum around
bell).
Class Scyphozoa-Life Cycle
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In some species the scyphistoma transforms
directly into a young medusa, (additional
evidence that the polyploid form was derived
from a larval stage in the evolution of the
cnidarians, or evidence that scyphozoans
evolved from anthozoans?).
In most species of scyphozoans young
medusae are budded off transversely from
the oral end of the scyphistoma which has
become a strobila.
Class Cubozoa.
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Formerly classified as order in
Scyphozoa. Cuboidal medusa with
tentacles hanging from corners of
medusa. Polyps very small and most
often unknown. Active swimmers and
feeders in warm tropical waters. Many
poisonous (see sea wasp above and in
text).
Class Cubozoa
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Class Anthozoa
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Either solitary or colonial polypoid
cnidarians in which the medusa
stage is completely absent.
Include sea anemones, corals, sea fans
and sea pansies. Largest Cnidarian
class with over 6000 species.
Class Anthozoa-Polyp Structure
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Many traits in common with Scyphozoa
(cellular mesoglea, gastrovascular
cavity divided into compartments,
cnidocytes in gastrovascular cavity and
gastrodermal gonads) so probably
more closely related to them than to
hydrozoa.
It is interesting to note that the margin
of the bell of the cubozoan medusa has
a fold of tissue analogous, but not
homologous, to the velum of
hydrozoans.
Class Anthozoa-Polyp Structure
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More complex than hydrozoan polyps.
Pharynx extends more than half way into
the gastrovascular cavity.
Mesenteries = longitudinal sheets that
divide the gastrovascular cavity into radiating
compartments. The edges of the mesenteries
bear nematocysts. May be for internal gas
exchange.
Anthozoa
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Anemones have no secreted skeleton,
are solitary and tend to be much larger
than the corals.
Corals are colonial and are often of
extreme ecological importance because
of their reef building.
-> prey killed and pushed into gut
• gastrodermal cells inject digestive enzymes
• adjacent cells absorb food or
• in colonies, moved in a slurry throughout
Bioluminescence in Paraphyllina intermedia (Scyphozoa)
Bi-radial Symmetry
plankton - Oahu, Hawai’i
http://www.biolum.org/marine/biolum2/middle/livinglights/lljellycontent.html
Colonial Hydroid colony and
Montastrea annularis polyps
Scyphozoan Medusa
a NW hydromedusa (Hydrozoa)
Fire Coral and Tube Anemone
http://ag.arizona.edu/~dgalbrai/jelly.gif
Phylum Ctenophora
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A minor group
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Compare to Cnidaria
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Fewer than 100 species
All are marine
Have 8 rows of comb-like plates for
locomotion
Nearly all free-swimming
Many are bioluminescent
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Similarities with Cnidaria
„ the “coelenterates”
„ mesoglea
„ symmetry – biradial and radial
„ Tissue Grade of Development
Differences:
„ gastrulation „ ctenes - plates of fused cilia
„ rows of ctenes = costae
„ Do not have nematocysts (one exception)
a benthic Ctenophore! (platyctene)…
The End.
…on a
starfish arm!
(Asteropsis carnifera)
Kailua-Kona,
Hawai’i