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.
Absence of a distinct head.
No segmentation.
Body wall contains interstitial cells and nematocysts.
Mouth surrounded by tentacles for defense and feeding.
Gastrovascular cavity for digestion; single opening serves as mouth and anus.
Extracellular digestion in the gastrovascular cavity.
Primitive nervous system as a nerve net.
Both asexual and sexual reproduction occur.

Key Terms

Nematocysts: Stinging cells containing a coiled thread used for capturing prey and defense.
Gastrovascular Cavity: A central cavity where digestion occurs, 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.
Interstitial Cells: Undifferentiated cells in the epidermis that can develop into other cell types.
Cnidoblasts: Cells that produce and contain nematocysts.