Chapter 9
Multicellular and Tissue Levels of
Organization
Phylum Porifera – The Sponges
Zoology
Rick Knowles
Liberty Senior High School
Origins of Multicellularity
• Why become multicellular?
• Division of labor, specialized cells with specific
functions.
• Two Hypotheses:
1. Colonial Hypothesis – a dividing protist
remained together.
2. Syncytial Hypothesis – formation of plasma
membranes in a protist may have produced a
small, multicellular organism.
The common ancestor
of living animals
• May have lived 1.2
billion–800 million years
ago
• May have resembled
modern
choanoflagellates,
protists that are the
closest living relatives of
animals.
• Identical to a type of
sponge cell – choanocytes
are used in feeding.
Single cell
Stalk
Characteristics of Porifera
• Porifera – means “pore
bearing”
• 9,000 + species, mostly
marine
• Asymmetrical Body
Plan
• Three Cell Types:
1. Pinococytes
2. mesenchyme cells
3. choanocytes
• Water canal system
• Cellular Organization,
but no tissue or organs
Images courtesy and copyright Dr. John Hooper,
Queensland Museum
Porifera Diversity – Three Classes
Class Demospongiae
Class Calcarea – composed of
calcite
Class Hexactinellida,
Staurocalyptus sp.
Images Courtesy:http://www.palaeontologie.geo.lmu.de/molpal/calcarea_introduction.html
More than Just Cute!
• Have a division of labor
• Pinacocytes – thin, flat cells that line
outer surface, may contract and
change shape of sponge or regulate
water entry – porocyte.
• Mesohyl – jellylike layer below
pinacocyte layer.
• Mesenchyme Cells – ameboid cells
moving about in mesohyl; for
reproduction, secreting structures,
food transport and storage.
• Choanocytes – flagellated cells below
mesohyl that line inner chamber(s);
create water current and filter
microscopic food.
http://www.ucmp.berkeley.edu/porifera/pororg.html
Sponge Skeleton
• Some species make
spicules – thorn-like
projections that provide
structural support and
protection; made of
calcium carbonate by
ameboid cells.
• Other species make a
fibrous protein of
collagen - spongin
Spicules
Sponges (Porifera)
Water Currents for Everything!
• Choanocytes use their flagella to create water
currents through external pores called – ostia
(sing. ostium,); incurrent pores.
• Bring food (bacteria, protists, etc.) and oxygen
and remove metabolic wastes from the center
of the sponge – spongocoel.
• Choanocytes use collar-like rings to filter food.
• Wastes and water flow out a central osculum
(plural, oscula); an excurrent pore.
Sponges (Porifera)
Sponge Anatomy
Choanocyte
1. Ascon Body Form
• Simplest canal
system with a
central spongocoel
lined with
Choanocytes and
with many ostia
opening directly
into spongocoel
• Least common
2. Sycon Body Form
• Sponge wall is
folded
• Water enters via
dermal pores
• Canal system with
a central
spongocoel into
which many radial
canals empty.
• Choanocytes line
radial canals
3. Leucon Body Form
• Branched incurrent
canals lead to
choanocyte-lined
chambers
• No spongocoel
• Increased surface
area = larger
volume of water
movement
• Most common
Let’s see them in action!
www.palaeontologie.geo.lmu.de/molpal/calcarea_introduction.html
Let’s Eat!
• Choanocytes filter microscopic food and trap in
collar.
• Placed into food vacuole and digested by lysosomes.
• Digested food is passed to amoeboid cells for
transport to other cells – beginnings of
specialization.
The Importance of Water Currents
• Respiration (gas exchange), Metabolism, and
Excretion all done by direct diffusion with
water.
• No nervous system – no responsiveness.
• Defenses – may produce some irritating
chemicals if touched; chemical defense
against predators, fish, sea stars, etc.
Reproduction
• Sponges are monoecious (both sexes in the same
individual – hermaphrodite).
• Do not self-fertilize. Why?
• Choanocytes become sperm.
• Other choanocytes and amoeboid cells become
eggs.
• Released from oscula and undergo external
fertilization.
• Larvae are free-swimming.
Free-swimming
larvae settle to the
bottom and ….
…become sessile
(attached to the
bottom) adults
Alternatives to Sex
• Asexual reproduction
from internal, resistant
capsules – gemmules.
– masses of ameboid cells
that are released when
parent dies.
– Dormant stage - resistant
to freezing and drying.
• Pieces broken off can
become a new sponge –
fragmentation.
• Grow new pieces –
budding.
And their good to eat, too!
Sponge Cake, Anyone?