- Multicellular life originated approximately 550 million years ago, arising quickly prior to the Precambrian-Cambrian boundary in an evolutionary explosion that resulted in most animal phyla. - There are two main hypotheses for the origins of multicellularity - the colonial hypothesis, where dividing cells remained together, and the syncytial hypothesis, where plasma membranes formed within the cytoplasm of a large protist. - Sponges are the simplest multicellular animals, consisting of loosely organized cells specialized for functions like pinacocytes on the outer surface, mesenchyme cells in the jellylike mesohyl, and choanocytes lining chambers for filter feeding. Sponges may have silica or calcium

1. ORIGINS OF MULTICELLULARITY
&
PHYLUM PORIFERA
By
SYED SHAKEEL SHAH
Department of Zoology
University of Narowal

2. ORIGINS OF MULTICELLULARITY
• Multicellular life has been a part of the earth’s history for
approximately 550 million years.
• Although this seems a very long time, it represents only
10% of the earth’s geological history.
• Multicellular life arose quickly in the 100 million years
prior to the Precambrian/Cambrian boundary, in what
scientists view as an evolutionary explosion.
• These evolutionary events resulted not only in the
appearance of all of the animal phyla recognized today,
but also 15 to 20 animal groups that are now extinct.
• Since this initial evolutionary explosion, most of the
history of multicellular life has been one of extinction.

3. • Colonial Hypothesis: The evolutionary events leading to
multicellularity are shrouded in mystery. Many zoologists
believe that multicellularity could have arisen as dividing cells
remained together, in the fashion of many colonial protists.
Although variations of this hypothesis exist, they are all treated
here as the colonial hypothesis.
FIG: (a) The colonial hypothesis. Multicellularity may have arisen when cells that a dividing
protist produced remained together. Cell invagination could have formed a second cell layer.
This hypothesis is supported by the colonial organization of some Sarcomastigophora. (The
colonial protist and the two-layered radial ancestor are shown in sectional views.)

4. Syncytial Hypothesis: A second proposed mechanism is called the
syncytial hypothesis. A syncytium is a large, multinucleate cell. The
formation of plasma membranes in the cytoplasm of a syncytial
protist could have produced a small, multicellular organism. Both
the colonial and syncytial hypotheses are supported by the colonial
and syncytial organization that occurs in some protist phyla.
(b) The syncytial hypothesis. Multicellularity could have arisen when plasma membranes
formed within the cytoplasm of a large, multinucleate protist. Multinucleate, bilateral ciliates
support this hypothesis.

5. ANIMAL ORIGINS
• A fundamental question concerning animal origins is whether animals are
monophyletic (derived from a single ancestor), diphyletic (derived from
two ancestors), or polyphyletic (derived from many ancestors).
• The view that animals are polyphyletic is attractive to a growing number
of zoologists.
• The nearly simultaneous appearance of all animal phyla in fossils from the
Precambrian/ Cambrian boundary is difficult to explain if animals are
monophyletic.
• If animals are polyphyletic, more than one explanation of the origin of
multicellularity could be possible, and more than one body form could be
ancestral.

6. • Conversely, the impressive similarities in animal cellular
organization support the view that all or most animals are
derived from a single ancestor.
• For example, asters form during mitosis in most animals,
certain cell junctions are similar in all animal cells, most
animals produce flagellated sperm, and the proteins that
accomplish movement are similar in most animal cells.
• These common features are difficult to explain, assuming
polyphyletic origins.
• If you assume one or two ancestral lineages, then only one or
two hypotheses regarding the origin of multicellularity can be
correct.

7. PHYLUM PORIFERA
• The Porifera (L. porus, pore fera, to bear), or sponges, are
primarily marine animals consisting of loosely organized
cells. The approximately nine thousand species of sponges
vary in size from less than a centimeter to a mass that
would more than fill your arms.
FIG: Phylum Porifera. Many sponges are brightly colored with hues of red, orange, green, or
yellow. (a) Verongia sp. (b) Axiomella sp.

8. Characteristics of the phylum Porifera include:
1. Asymmetrical or 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

9. CELL TYPES, BODY WALL, AND SKELETONS
• In spite of their relative simplicity, sponges are more than
colonies of independent cells.
• As in all animals, sponge cells are specialized for
particular functions.
• This organization is often referred to as division of labor.

10. • Thin, flat cells, called pinacocytes, line the outer surface of a sponge.
• Pinacocytes may be mildly contractile, and their contraction may change the
shape of some sponges.
• In a number of sponges, some pinacocytes are specialized into tubelike,
contractile porocytes, which can regulate water circulation.
• Openings through porocytes are pathways for water moving through the
body wall.
• Just below the pinacocyte layer of a sponge is a jellylike layer called the
mesohyl (Gr. meso, middle hyl, matter).
• Amoeboid cells called mesenchyme cells move about in the mesohyl and are
specialized for reproduction, secreting skeletal elements, transporting and
storing food, and forming contractile rings around openings in the sponge
wall.

11. • Below the mesohyl and lining the inner chamber(s) are
choanocytes, or collar cells. Choanocytes (Gr. choane, funnel
cyte, cell) are flagellated cells that have a collarlike ring of
microvilli surrounding a flagellum.
• Microfilaments connect the microvilli, forming a netlike mesh
within the collar.
• The flagellum creates water currents through the sponge, and
the collar filters microscopic food particles from the water.
• The presence of choanocytes in sponges suggests an
evolutionary link between the sponges and a group of protists
called choanoflagellates.

12. FIG: Morphology of a Simple Sponge. (a) In this example, pinacocytes form the outer body
wall, and mesenchyme cells and spicules are in the mesohyl. Porocytes that extend through
the body wall form ostia. (b) Choanocytes are cells with a flagellum surrounded by a collar of
microvilli that traps food particles. Food moves toward the base of the cell, where it is
incorporated into a food vacuole and passed to amoeboid mesenchyme cells, where digestion
takes place. Blue arrows show water flow patterns. The brown arrow shows the direction of
movement of trapped food particles.

13. Skeleton
• Sponges are supported by a skeleton that may
consist of microscopic needlelike spikes called
spicules.
• Spicules are formed by amoeboid cells, are
made of calcium carbonate or silica, and may
take on a variety of shapes.
• Alternatively, the skeleton may be made of
spongin (a fibrous protein made of collagen),
which is dried, beaten, and washed until all
cells are removed to produce a commercial
sponge.
• The nature of the skeleton is an important
characteristic in sponge taxonomy.
FIG: Sponge Spicules. Photomicrograph of a
variety of sponge spicules (150).

14. Thank You…!