First Multicellular Animals

Question 1

What is the advantage of being multicellular?

Answer 1

Dividing a mass into smaller units (multicellularity) rather than increasing the mass of a single cell increases the surface area available for metabolic activities.

Question 2

What are the distinct features of the two hypotheses that explain the evolution of multicellularity?

Answer 2

Syncytial ciliate hypothesis: Origin from a single-celled ciliate with multiple nuclei, which became compartmentalized. Ancestral form was bilaterally symmetrical. BUT embryology of current metazoans does not show compartmentalization, this theory implies that radial symmetry of current metazoans is a derived character, and it does not explain flagellated cells and larvae of sponges.

Colonial flagellate hypothesis: Origin from a colony of flagellated cells. Individual cells became differentiated for specific functions. Ancestral form was radially symmetrical.

Question 3

What was the advancement of sponges over protozoans?

Answer 3

Sponges are multicellular as opposed to unicellularity of protozoans

Question 4

How does a sponge feed, get oxygen, and excrete wastes?

Answer 4

It draws in water through its pores and removes oxygen and food particles via cells called choanocytes. Cells called archeocytes remove mineral wastes that threaten to block the ostia, transport them to the mesohyl and dump them into the outgoing water current.

Question 5

What’s the route of water flow in the sponge? Which cells are critical in collecting nutrients from the water and metabolizing them?

Answer 5

The route of water flow is through its pores. Choanocytes are the cells which are vital in collecting nutrients from the water and metabolizing.

Question 6

Three different canal systems? What’s the rationale behind the advancement in the type of canal system?

Answer 6

1. Asconoids (simplest)- Water drawn through pores called ostia into a large chamber called the spongocoel lined by choanocytes. Used water is expelled thru single large opening called osculum- limited surface area for filtering water.
2. Syconoids- Spongocoel folded into choanocyte-lined radial canals, increasing the surface area available for absorption
3. Leuconoid (most complex)- Surface area for food collection further increased by formation of chambers lined with choanocytes. Water enters and leaves through incurrent and excurrent canals (respectively)

Question 7

What are the defining characters of a sponge?

Answer 7

1. Multicellular but a loose aggregation of cells
2. Body with pores that draws water and removes food and oxygen from it. All aquatic, mostly marine
3. Sessile. Radial or asymmetric.
4. Intracellular digestion. Excretion and respiration by diffusion.
5. No nervous system
6. Monoecious (hermaphrodytic). Reproduction both asexual and sexual. External fertilization. Free-swimming flagellated larvae.