Chapter 9 Multicellular and Tissue Levels of

Organization

Phylum Porifera – The Sponges

ZoologyLiberty 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 – used in feeding.

Single cell

Stalk

Characteristics of Porifera• Porifera – means “pore

bearing”• 9,000 + species, mostly

marine• Asymmetrical Body

Plan• Three Cell Types:

pinococytes, mesenchyme cells, and choanocytes

• Water canal system• Cellular Organization,

but no tissue or organsImages courtesy and copyright Dr. John Hooper, Queensland Museum

Porifera Diversity – Three Classes

Class Calcarea – composed of calcite

Class Hexactinellida, Staurocalyptus sp.

Images Courtesy:http://www.palaeontologie.geo.lmu.de/molpal/calcarea_introduction.html

Class Demospongiae

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 pinacotye 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 exteranl 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.

• Gemmules – are 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?