Advent of Multicellularity
Chapter 12
Introduction to
Metazoans:
Phylum Porifera
Nature’s experiments with larger organisms
without cellular differentiation are limited.
Increasing the size of a cell causes problems of
exchange; multicellularity avoids surface-tomass problems.
Cell assemblages in sponges are distinct from
other metazoans; molecular evidence shows
common ancestry.
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Positions in the Animal Kingdom
Multicellular organisms are divided into three grades:
Mesozoa
Parazoa (phylum Porifera, phylum Placozoa)
Eumetazoa (all other animal phyla)
Mesozoa and Parazoa are multicellular but lack germ
layers of Eumetazoa.
They have a cellular level of organization
Mesozoans are entirely parasitic but have a complex reproductive
cycle.
Placozoans are composed of two layers of epithelia with fluid and
fibrous cells between them.
Sponges are more complex and organized into incipient tissues
with low integration.
Syncytial Ciliate Theory
The body form resembled modern ciliates with a
tendency toward bilateral symmetry.
This would resemble flatworms, but their
embryology fails to show cellularization, and
flatworms have flagellated sperm.
This would mean that radial cnidarians had a
bilateral ancestor.
Origin of Metazoa
Three Theories of Unicellular Origin of
Metazoans
Metazoans arose from syncytial (multinucleate)
ciliated forms
Metazoans arose from a colonial flagellated form
Metazoans are polyphyletic, derived from more than
one group of unicellular organisms.
Colonial Flagellate Theory
Haeckel first proposed this in 1874.
As cells in a colony became more specialized,
the colony became dependent on them.
The model is free-swimming planula larvae of
cnidarians; it is radially symmetrical.
Bilateral symmetry would have evolved when
the planula larvae adapted to crawling on the
floor.
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Polyphyletic Origins Theory
Sponges, cnidarians, ctenophores and remaining
eumetazoans each evolved independently.
This would not conform to any lineage
suggested above.
Phylum Mesozoa
Named by van Beneden in 1876; he believed mesozoa
was link between protozoa and metazoa.
All are parasites in marine invertebrates.
They are small, made of 20-30 cells in two layers but
these are not germ layers.
Some researchers believe they represent degenerate
flatworms and place them in Platyhelminthes.
Others place them close to protozoa, perhaps ciliates.
Phylum Porifera
Porifera means "pore-bearing"; their sac-like bodies are
perforated by many pores.
They are sessile and depend on water currents to bring
in food and oxygen and carry away wastes.
Molecular Evidence
Small subunit ribosomal RNA and biochemical
pathways support the colonial flagellate
hypothesis.
Metazoans appear to be monophyletic and
arising from choanoflagellates.
The syncitial ciliate hypothesis is excluded
because Metazoa are closer to eukaryotic algae
and higher plants than to ciliates.
Phylum Placozoa
Composed entirely of one marine species Trichoplax
adhaerens
It has no symmetry, and no muscular or nervous organs.
Cell layers include: a dorsal epithelium, a thick ventral
epithelium of monociliated cells and nonciliated gland
cells, and a space between containing fluid and fibrous
cells.
Molecular evidence suggests they are a sister group of
Cnidaria.
General Features
Body is a mass of cells embedded in gelatinous matrix
and stiffened by spicules of calcium carbonate or silica
and collagen.
They have no organs or tissues; cells are somewhat
independent.
They have no nervous or sense organs and have
simplest of contractile elements.
They are aside from the mainstream of animal
evolution; thus they are often called Parazoa.
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General Features (continued)
Sponges vary from a few millimeters to 2 meters
across; they vary greatly in shape and color.
Most of the 5000 species are marine; about 150 are
freshwater.
Morphology changes with substratum, calmness of
water, etc.
Sponges are ancient; fossils extend to Cambrian or
earlier.
There are three types of canal systems.
Form and Function
Body openings consist of small incurrent pores or ostia
and a few excurrent oscula.
Openings are connected by a system of canals; water
passes from ostia to osculum.
Choanocytes or flagellated collar cells line some of the
canals.
They keep the current flowing by beating of flagella.
They trap and phagocytize food particles passing by.
The framework of the sponge is composed of needlelike calcareous or siliceous spicules or organic spongin
fibers.
Fig. 12.6
Asconid: Flagellated Spongocoels
Asconoids are simplest; they are
small and tube-shaped.
Water enters a large cavity, the
spongocoel, lined with choanocytes.
Choanocyte flagella pull water
through.
All Calcarea are asconoids:
Leucosolenia and Clathrina are
examples.
Three types of canal systems
Syconoids: Flagellated Canals
They resemble asconoids but are
bigger with a thicker body wall.
The wall contains choanocyte-lined
radial canals that empty into the
spongocoel.
Water entering filters through tiny
openings called prosopyles.
The spongocoel is lined with
epithelial cells rather than
choanocytes.
Food is digested by choanocytes.
Syconoids (continued)
Flagella force the water through internal
pores called apopyles into the spongocoel
and out the osculum.
They pass through an asconoid stage in
development but do not form highly
branched colonies.
The flagellated canals form by
evagination of the body wall; this is
developmental evidence of being derived
from asconoid ancestors.
Classes Calcarea and Hexactinellida have
species that are syconoid; the genus Sycon
(Grantia) is an example.
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Three types of canal systems
Fig. 12.8
Leuconoids: Flagellated
Chambers
These are most complex and are
larger with many oscula.
Clusters of flagellated chambers are
filled from incurrent canals, and
discharge to excurrent canals.
Most sponges are leuconoid; it is seen
in most Calcarea and in all other
classes.
The leuconoid system has evolved
independently many times in
sponges.
This system increases flagellated
surfaces compared to volume; more
collar cells can meet food demands.
Types of Cells
Sponge cells are arranged in
a gelatinous matrix called
mesohyl.
Pinacocytes
These cells form the
pinacoderm; they are flat
epithelial-like cells.
Pinacocytes are somewhat
contractile.
Some are myocytes that help
regulate flow of water.
Archaeocytes
These cells move about in the
mesohyl.
They phagocytize particles in
the pinacoderm.
They can differentiate into any
other type of cell.
Those called sclerocytes
secrete spicules.
Spongocytes secrete spongin.
Collencytes secrete fibrillar
collagen.
Lophocytes secrete lots of
collagen but may look like
collencytes.
Choanocytes
These are oval cells with one end
embedded in mesohyl.
The exposed end has a flagellum
surrounded by a collar.
A collar is made of adjacent
microvilli forming a fine filtering
device to strain food.
Particles too large to enter the
collar are trapped in mucous and
moved to the choanocyte where
they are phagocytized.
Food engulfed by choanocytes is
passed to neighboring
archaeocytes for digestion.
Fig. 12.10
Types of Skeletons
Collagen fibrils are found throughout intercellular
matrix of sponges.
Various Demospongiae secrete a form of collagen
called spongin.
Demospongiae also secrete siliceous spicules.
Calcareous sponges secrete spicules of crystalline
calcium carbonate.
Glass sponges have siliceous spicules with six rays.
Spicule patterns are important taxonomic features.
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Sponge Physiology
Filtration Rates
Leuconia, a small sponge, has 81,000 incurrent
canals.
It would have more than two million flagellated
chambers.
Expulsion of water carries wastes some distance
away.
Some large sponges can filter 1500 liters of water a
day.
Sponge Physiology (continued)
Particles are filtered nonselectively; choanocytes
phagocytize 80%.
Digestion is completely intracellular, primarily
by archaeocytes.
There are no excretory or respiratory organs;
diffusion suffices.
The only movements are very slow opening and
closing of pores; nerve cells have not been
demonstrated.
Sexual Reproduction
Most are monoecious with both male and
female sex cells in one individual.
Sperm arise from transformed choanocytes.
In some Demospongiae and Calcarea, oocytes
develop from choanocytes; others derive them
from archaeocytes.
Sponges provide nourishment to the zygote until
it is released as a ciliated larva.
Asexual Reproduction
External buds are small
individuals that break off
after attaining a certain size.
Internal buds or gemmules
are formed by archaeocytes
that collect in mesohyl and
are coated with tough
spongin and spicules; they
survive drought, freezing, etc.
Sexual Reproduction (continued)
In some, when one sponge releases sperm,
they enter the pores of another.
Choanocytes phagocytize the sperm and
transfer them to carrier cells that carry sperm
through mesohyl to oocytes.
Some release both sperm and oocytes into
water.
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Development
The free-swimming larva of sponges is a solid
parenchymula.
Regeneration and Somatic Embryonogenesis
Sponges can regenerate wounded portions.
Sponge fragments aggregate into new structures,
this is somatic embryogenesis.
Class Calcarea
These are calcareous sponges with spicules of
calcium carbonate.
The spicules are straight or have three or four
rays.
Most are small sponges with tubular or vase
shapes.
Asconoid, syconoid and leuconoid forms all
occur.
Calcarea and some Demospongiae have strange
development.
A hollow amphiblastula develops with
flagellated cells toward the interior.
The blastula then turns inside out
(inversion).
Flagellated cells or micromeres of the
larva are at one end; larger nonflagellated macromeres are at the other
end.
Macromeres overgrow the micromeres.
Flagellated micromeres become
choanocytes, archaeocytes and
collencytes; nonflagellated cells give
rise to pinacoderm and sclerocytes.
Representative Classes
Class Calcarea (Calcispongiae)
Class Hexactinellida (Hyalospongiae)
Class Demospongiae
Class Hexactinellida
These are glass sponges with six-rayed spicules of
silica.
Nearly all are deep-sea forms; most are radially
symmetrical.
Root spicules attach them to the substrate.
Their spicular network forms a network; a trabecular
net of living tissue made of a fusion of pseudopodia of
archaeocytes forms the chambers opening to the
spongocoel.
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Class Hexactinellida (continued)
They lack a pinacoderm or
gelatinous mesohyll and
myocytes are absent.
The body of hexactinellids is
composed of a single,
continuous synctial tissue
called a trabecular reticulum.
(This is the largest continuous
syncytial tissue known in
Metazoa.)
Collar bodies do not participate
in phagocytosis; that process is
accomplished by the primary
and secondary reticula.
Class Hexactinellida (conclusion)
Chambers appear to correspond to both syconoid
and leuconoid types.
They are adapted to a deep-water habitat with a
large and easy flow of water.
Some scientists advocate placing hexactinellids
in a subphylum separate from other sponges.
Class Demospongiae
This class contains 95% of living sponge species.
Spicules are siliceous but not six rayed; they may be
absent or bound together by spongin.
All are leuconoid and all are marine except for
Spongillidae, the freshwater sponges.
Freshwater sponges flourish in summer and die in late
autumn, leaving gemmules.
Marine demosponges are highly varied in color and
shape.
Bath sponges belong to a group that lacks siliceous
spicules but have spongin skeletons.
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Phylogeny and Adaptive Radiation
Sponges appeared before the Cambrian.
Glass sponges rapidly expanded in the Devonian.
One theory is that sponges arose from
choanoflagellates; however, some corals and
echinoderms also have collar cells, and sponges acquire
them late in development.
Molecular rRNA evidence suggests a common ancestor
for choanoflagellates and metazoans and that sponges
and Eumetazoa are sister groups with Porifera splitting
off before radiates and placozoans.
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