Sponges and
Placozoans
Chapter 12
Origin of Metazoa
 Evolution of the eukaryotic cell was followed by
diversification into many lineages including:
 Modern protozoans
 Plants
 Fungi
 Animals
 Multicellular animals are called metazoans.
Dendrogram of Major Phyla
Echinodermata
Vertebrata
Nematoda
Protochordata
Hemichordata
Apicomplexa
Uniramia
Actinopoda
Ciliophora
Euglenozoa
Rhizopoda
Granuloreticulosa
Crustacea
Chelicerata
Priapulida
Kinorhyncha
Loricifera
Annelida
Mollusca
Bryozoa
Brachiopoda
Phoronida
Platyhelminthes
Rotifera
Cnidaria
Porifera
Choanoflagellates
 Choanoflagellates
are solitary or
colonial protozoans
with a flagellum
surrounded by a
collar of microvilli.
Choanoflagellates
 Choanoflagellates
resemble sponge feeding
cells (choanocytes).
 Scientists are studying
colony formation and cellto-cell communication in
choanoflagellates in
search of clues to the
evolution of multicellularity.
Colonial Flagellate
Hypothesis
 Colonial Flagellate Hypothesis – metazoans
descended from ancestors characterized by a
hollow, spherical colony of flagellated cells.
 Individual cells became specialized for different
functions.
 Radially symmetrical, similar to a blastula.
 First proposed by Haeckel in 1874
Phylum Porifera
 Sponges, Phylum Porifera, are multicellular
heterotrophs.
 They are asymmetrical.
 They lack true tissues and organs.
 Molecular evidence suggests they do share a
common ancestor with other animals.
 Kingdom Animalia is monophyletic.
Phylum Porifera
 Sponges are sessile
animals that have a
porous body and
choanocytes.
 Supported by a
skeleton of tiny
needlelike spicules
and protein.
 They live in both
fresh and marine
waters.
Phylum Porifera
 Sponges range in size and shape.
 Up to 2 meters in diameter!
 Encrusting, boring, finger, tube or vase shaped.
Neighbors
 Many organisms,
including crabs,
nudibranchs, mites,
bryozoans, and fish
live as commensals
or parasites in
sponges.
Skeletal Framework
 The skeletal framework
of a sponge may be
fibrous or rigid.
 The fibrous part comes
from collagen fibrils in
the intercellular matrix.
 Spongin
 Rigid skeletons consist
of needlelike spicules.
 Calcareous
 Siliceous
 Composition and
shape the spicules
forms the basis of
sponge classification.
Suspension Feeders
 Sponges are suspension feeders capturing
food particles suspended in the water that
passes through their body.
Suspension Feeders
 Water flows in
through incurrent
pores called dermal
ostia.
 It flows past the
choanocytes where
food particles are
collected on the
choanocyte collar.
Suspension Feeders
 Choanocytes take in small particles by
phagocytosis. Protein molecules are taken in
by pinocytosis.
 Sponges can also absorb nutrients dissolved in
the water.
Canal Systems
 Asconoid – the
simplest canal
system.
 Choanocytes line the
spongocoel.
 Water enters through
the ostia and exit
through the large
osculum.
 Usually tube shaped.
 Found only in the
Class Calcarea.
Canal Systems
 Syconoid – tubular
body and singular
osculum like
asconoids.
 The walls of the
sponge are folded to
form choanocyte
lined canals.
 Increased area for
feeding.
 Class Calcarea.
Canal Systems
 Leuconoids – most
complex, permits an
increase in sponge
size.
 Choanocytes line
the walls of small
chambers where
they can filter all the
water that flows
through.
 Most sponges.
Types of Cells
 Absence of tissues & organs means that
fundamental processes occur on the cellular
level.
 Respiration and excretion occur by diffusion in
each cell.
 Mesohyl is the gelatinous matrix containing
skeletal elements & amoeboid cells.
Types of Cells
 Choanocytes, flagellated collar cells, generate
a water current through the sponge and ingest
suspended food.
Types of Cells
 The choanocytes pass
food particles to
archaeocyte cells for
digestion.
 Digestion occurs entirely
within cells, there is no
gut.
 Other cell types secrete
spicules (sclerocytes),
spongin (spongocytes), &
collegen (collenocytes).
Types of Cells
 Pinacocytes are
thin, flat, epithelialtype cells that cover
the exterior and
some interior
surfaces of the
sponge.
 Almost a true tissue.
Reproduction
 Sponges have remarkable
regeneration capabilities.
 Regeneration following
fragmentation is a form of
asexual reproduction.
 External buds can break
off to form new sponges.
 Internal buds (gemmules)
in freshwater sponges can
remain dormant in times of
drought.
Reproduction
 Most sponges are hermaphrodites meaning
that each individual functions as both male and
female.
 Monoecious
 Gametes are derived from choanocytes or
sometimes archaeocytes.
Reproduction
 Most sponges are viviparous.
 After fertilization, the zygote is retained and is
nourished by the parent. Ciliated larvae are
later released.
 Some are oviparous releasing gametes into
the water.
Reproduction
 Sponges in the class Calcarea and a few
Demospongiae have an unusual developmental pattern
where the embryo turns inside out.
 Flagellated cells become choanocytes & archaeocytes.
 Larger cells become pinacocytes.
Class Calcarea
 Calcareous sponges (Class Calcarea) have
spicules composed of calcium carbonate.
 Small, usually vase shaped.
 Asconoid, syconoid, or leuconoid in structure.
Class Hexactinellida
 Glass sponges (Class Hexactinellida) are mostly deep
sea forms.
 Spicules are six-rayed and made of silica.
 Hexactinellids lack a pinacoderm or gelatinous
mesohyll.
 Chambers appear to correspond to both syconoid and
leuconoid types.
Class Hexactinellida
 Some advocate placing
hexactinellids in a subphylum
separate from other sponges.
 Trabecular reticulum made
of a fusion of archaeocyte
pseudopodia - forms the
chambers opening to
spongocoel.
 Trabecular reticulum is
largest continuous
syncytial tissue known in
Metazoa.
 Choanoblasts are associated with flagellated chambers.
 Collar bodies do not participate in phagocytosis – this is
the function of the primary and secondary reticula.
Class Demospongiae
 Class Demospongiae
contains most of the
sponge species.
 Spicules are siliceous,
but not six-rayed.
 Spicules may be bound
together by spongin, or
absent.
 All leuconoid, mostly
marine.
Class Homoscleromorpha
 A fourth class, Homoscleromorpha, was
formed to contain sponges without a skeleton
or with siliceous spicules without an axial
filament.
Cladogram of Sponge
Classes
Phylogeny and Adaptive
Diversification
 Sponges appeared before the Cambrian.
 Glass sponges expanded in the Devonian.
 One theory - sponges arose from
choanoflagellates.
 However, some corals and echinoderms also have
collar cells, and sponges acquire them late in
development.
Phylogeny and Adaptive
Diversification
 Molecular rRNA evidence suggests a common
ancestor for choanoflagellates and metazoans.
 Sponges and Eumetazoa are sister groups with
Porifera splitting off before radiates and placozoans.
Phylum Placozoa
 Trichoplax adhaerens
is the sole species of
phylum Placozoa
(marine).
 No symmetry
 No muscular or nervous
organs
 Placozoans glide over
food, secrete digestive
enzymes, and absorb
nutrients.
Phylum Placozoa
 Cell layers
 Dorsal epithelium
 Thick ventral epithelium of monociliated cells and
nonciliated gland cells.
 Space between the epithelia contain fibrous “cells”
within a contractile syncytium.
 Grell considers it diploblastic.
 Dorsal epithelium represents ectoderm and ventral
epithelium represents endoderm.