Phylum Coelentrata (Cnidaria) Cheat Sheet

Phylum Coelentrata, also known as Cnidaria, are primitive, radially symmetrical aquatic animals characterized by diploblastic tissue, a gastrovascular cavity, and stinging cells called nematocysts.

Core Principles

Marine and freshwater inhabitants.
Primitive metazoans with low tissue organization.
Can be solitary or colonial, sessile or free-swimming.
Radially symmetrical body with oral and aboral axes.
Diploblastic: outer ectoderm, inner endoderm, with mesoglea in between.
Head and segmentation are absent.
Body wall contains interstitial cells and stinging nematocysts.
Mouth surrounded by tentacles for defense and feeding.
Gastrovascular cavity for digestion and circulation; single opening serves as mouth and anus.
Extracellular digestion occurs in the gastrovascular cavity.
Primitive nervous system: a diffuse nerve net.
Reproduce both asexually (budding, regeneration, splitting) and sexually.

Key Terms

Nematocysts: Stinging cells containing a coiled thread used for prey capture, defense, and attachment.
Gastrovascular Cavity: A central cavity responsible for digestion and circulation, with a single opening (mouth).
Diploblastic: Having two primary germ layers: ectoderm and endoderm.
Radial Symmetry: Body parts arranged around a central axis.
Mesoglea: A jelly-like layer between the ectoderm and endoderm.
Cnidoblasts: Cells that produce and contain nematocysts.
Hypostome: A conical structure at the oral end, bearing the mouth and surrounded by tentacles.
Pedal Disc: A structure at the aboral end, often used for attachment.

Real World Examples

Hydra: A common freshwater cnidarian exhibiting budding, regeneration, and various locomotion methods like gliding and looping.
Jellyfish: Free-swimming marine cnidarians with a prominent medusa stage, known for their stinging tentacles.
Corals: Sessile, colonial cnidarians that secrete a calcium carbonate exoskeleton, forming reefs.
Sea Anemones: Sessile, predatory cnidarians with a flower-like appearance, using tentacles to capture prey.