Sponges, Corals, and Pre-Bilaterian Life Flashcards
What is a phylum in biological classification
a major branch of the animal tree of life defined by a unique body plan and shared organisation
Which early-diverging phyla are considered pre-bilaterian
Porifera (sponges), Placozoa, Cnidaria, and Ctenophora
Does early divergence mean primitive
No, early branching just means they diverged earlier, not that they are less evolved
What are the key cell types in sponges
Pinacocytes (outer layer)
Inner cells (involved in feeding)
Pore cells
Choanocytes (collar cells that move water and capture bacteria)
How do choanocytes in sponges work
They form a sheet, draw in water using flagella, and trap food particles
What do choanocytes resemble and why is it significant
They resemble choanoflagellates, suggesting possible evolutionary links or convergent evolution
What is the body structure of Placozoa
flat, amoeba-like organism with two layers of cells
Upper layer: ciliated
Lower layer: includes secretory gland cells
How does Placozoa feed
Crawls over algae and secretes digestive enzymes
What groups are included in Cnidaria
Hydrozoa, Scyphozoa, Cubozoa, Anthozoa
What type of symmetry do cnidarians have
Radial Symmetry
What are the two main life forms in Cnidaria
Polyp (sessile)
Medusa (free-swimming, more contractile)
What is mesoglea
A jelly-like layer between the two cell layers in cnidarians
What cells enable movement in cnidarians
Contractile cells in both outer and inner layers; no true muscle cells
What are nematocysts
Stinging organelles used to capture prey
How do nematocysts work
- Thread pierces prey
- Osmotic swelling triggers ejection
- Elastic recoil shoots venom tube
- Stylets punch a hole
- Tube everts and injects venom
What is a colonial cnidarian example
The Portuguese man o’ war (a hydrozoan) and coral colonies
What is the body plan of ctenophores
Two-layered, radially symmetric predators
How do ctenophores capture prey
Using colloblasts — sticky cells
How do ctenophores move
Swim using rows of cilla which beat and create a water current
What are the major problems in early animal phylogeny
- Uncertainty about the branching order of
early-diverging phyla - Xan groups don’t clearly fit in - have a front and back, but no CNS or through-gut
- Long Branch Attraction (LBA) - lots of mutations in some lineages (e.g. Xans) obscure evolutionary relationships
