Chapter 9
Multicellular and Tissue
Levels of Organization
Evolutionary Perspective
• Porifera
–
–
–
–
No tissues
Division of labor among independent cells
Independent origin from common animal ancestor
Choanoflagellate protists (?)
• Cnidaria and Ctenophora
– Tissue level organization
– Independent origins from common animal ancestor
– Choanoflagellate protists (?)
• Origins of Multicellularity
– At least 800 million years—Precambrian
– Colonial hypothesis
– Syncytial hypothesis
Figure 9.1 Evolutionary relationships of the
Porifera, Cnidaria, and Ctenophora.
Figure 9.2 Two hypotheses regarding the
origin of multicellularity. (a) Colonial
hypothesis. (b) Syncytial hypothesis.
Phylum Porifera
1. Asymmetrical or superficially
radially symmetrical
2. Three cell types: pinacocytes,
mesenchyme cells, and
choanocytes
3. Central cavity, or a series of
branching chambers, through which
water circulates during filter
feeding
4. No tissues or organs
Table 9.1
Cell Types, Body Wall, and
Skeletons
•
Pinacocytes
•
Mesohyl
•
Mesenchyme cells
•
Choanocytes
•
Skeleton
– Outer surface
– Some contractile, others may be specialized into porocytes
– Jellylike middle layer
– Amoeboid cells
– Reproduction, secreting skeletal elements, transporting and storing
food, form contractile rings
– Flagellated
– Collarlike ring of microvilli
– Water currents for filter feeding
– Spicules
– Spongin
Figure 9.4 Morphology of a simple sponge.
Figure 9.5 Sponge spicules.
Figure 9.6 Water Currents and Body Forms.
• Complex sponges have increased surface
area for filtering large volumes of water.
Maintenance functions
• Filter feeding
– Bacteria, algae, protists, suspended organic
matter
– Trapped in choanocyte collar and incorporated
into food vacuole
– Digestion by lysosomal enzymes and pH
changes
• Nitrogeneous waste removal and gas
exchange
– Diffusion
• Coordination
– Responses of individual cells (some
coordination)
Reproduction
• Monoecious
• Choanocytes (and sometimes
ameboid cells) lose collars and
flagella and undergo meiosis.
• External fertilization and
planktonic larvae in most
• Asexual reproduction
– Gemmules
– Freshwater and some marine
Figure 9.7
Development of sponge
larval stages.
(a)Parenchymula larva.
(b) Amphiblastula
larva.
(c) Gemmule.
(d)
Phylum Cnidaria
1. Radial symmetry or modified as biradial
symmetry
2. Diploblastic, tissue-level organization
3. Gelatinous mesoglea between the
epidermal and gastrodermal tissue
layers
4. Gastrovascular cavity
5. Nerve cells organized into nerve net
6. Specialized cells, called cnidocytes,
used in defense, feeding, and
attachment
The Body Wall
• Epidermis
– Outer cellular layer
– Ectodermal origin
• Gastrodermis
– Inner cellular layer
– Endodermal origin
• Mesoglea
– Jellylike
– Cells present but origins are epidermal
or endodermal
Figure 9.8 Body wall of a cnidarian.
Nematocysts
• Cnidocytes
– Epidermal or gastrodermal cells that
produce cnida
– 30 types
• Nematocysts used in food gathering and
defense
Figure 9.9 Cnidocyte structure and nematocyst discharge.
Figure 9.10 The generalized cnidarian life cycle involves
alternation between a sexual medusa stage and an asexual polyp
stage.
Maintenance Functions
• Gastrovascular cavity
–
–
–
–
–
Digestion
Gas exchange
Excretion
Reproduction
Hydrostatic skeleton
• Support and movement
• Epitheliomuscular cells act against waterfilled cavity.
• Nerve net coordinates body
movements.
Reproduction
• Medusa
– Dioecious
– External fertilization most common
– Planula larva
• Polyp
– Budding produces miniature
medusae.
Class Hydrozoa
• Mostly marine
• Some freshwater
• Unique features
– Nematocysts only epidermal
– Gametes epidermal and released to
outside of body
– Mesoglea largely acellular
– Medusae with velum
Figure 9.11 Obelia structure and life cycle.
Figure 9.12 Gonionemus medusa. The velum is
unique to members of the Hydrozoa.
Class Staurozoa
Figure 9.13 Members of the class Staurozoa are
marine and lack a medusa stage. Lucernaria janetae
is shown here.
Class Scyphozoa
• Marine
• Medusa dominant in life history
– Lacks velum
• Cnidocytes epidermal and
gastrodermal
• Gametes gastrodermal
– Dioecious
Figure 9.14 Representative scyphozoans (a) Mastigias
qinquecirrha and (b) Aurelia labiata.
(a)
(b)
Figure 9.15
Structure of the
scyphozoan
medusa of
Aurelia.
Figure 9.16 Aurelia life history.
Class Cubozoa
• Cuboidal medusa
• Tentacles hang
from corners
• Tropical
• Dangerous
nematocysts
Figure 9.17 The sea wasp,
Chironex fleckeri.
Class Anthozoa
Colonial or solitary
Lack medusa
Cnidocytes lack cnidocils
Anemones and corals
Mouth leads to pharynx
Mesenteries divide gastrovascular
cavity and are armed with
nematocysts.
• Mesoglea with ameboid
mesenchyme cells
•
•
•
•
•
•
Figure 9.18 (a) The giant sea anemone (Anthopleura
xanthogrammica) and (b) a sea anemone (Callictis parasitical)
living in a mutualistic relationship with a hermit crab.
Figure 9.19 The structure of the anemone,
Metridium sp.
Reproduction
• Asexual
– Pedal laceration
– Longitudinal or transverse fission
• Sexual
– Monoecious or dioecious
– External fertilization produces planula.
– Monoecious species
• Protandry
– Male gametes mature first.
Corals
• Stony
– Reef forming
– Lack siphonoglyphs
– Cuplike calcium carbonate
exoskeleton
– Asexual budding expands colony.
– Symbiotic relationship with
zooxanthellae
Figure 9.20 A
stony coral polyp in
its calcium
carbonate
skeleton.
Corals
• Octacorallian
– Warm waters
– Eight pinnate tentacles
– Eight mesenteries
– Internal protein or calcium carbonate
skeleton
Figure 9.21 Octacorallian corals (a) Ptilosaurus gurneyi
and (b) Gorgonia ventalina.
Phylum Ctenophora
1. Diploblastic or possibly
triploblasitic
2. Biradial symmetry
3. Gelatinous, cellular mesoglea
4. True muscle cells
5. Gastrovascular cavity
6. Nerve net
7. Colloblasts
8. Eight comb rows
Table 9.3
Phylum Ctenophora
• Cellular mesoglea and true muscle
cells suggest that members may be
triploblastic.
• Locomotion by bands of cilia are
called comb rows.
• Tentacles contain adhesive cells
called colloblasts that capture prey.
• Monoecious with gastrodermal
gonads
– External fertilization leads to flattened
larval stage.
Figure 9.22 (a) The
bioluminescent
ctenophoran Mnemiopsis
sp. (b) The structure of
Pleurobranchia. (c)
Colloblast structure.
(a)
Further Phylogenetic
Considerations
• Porifera
– Oldest fossil deposits
– Choanoflagellate ancestors
– Increases surface-to-volume ratio in syconoid and
leuconoid body forms evolved in response to
selection for increased size.
• Cnidaria
– Radially symmetrical ancestor
• Minority view suggest bilateral ancestor.
– Molecular data and morphology suggests
relationships shown in figure 9.23.
• Ctenophora
– Relationships to other groups uncertain but
probably distant
Figure 9.23 Cladogram showing cnidarian taxonomy.