Cnidarians – Stinging Celled Animals Section 28.2

Body Structure Radial symmetry Two layers of cells that form tissues:  Outer layer from the ectoderm  Inner layer from the endoderm Tentacles surround the opening to the gastrovascular cavity  Contain cnidocytes – stinging cells – with nematocysts that have toxins for defense or stinging prey Tentacles draw prey into gastrovascular cavity Food digestion begins with enzymes in the gastrovascular cavity (extracellular digestion) and ends inside the cells Wastes leave through the same opening

Two Body Forms 1. Medusa Free-floating Jellylike Usually umbrella-shaped 2. Polyp Tube-like Attached to rock or other object Some cnidarians spend whole life in one form or the other; Other cnidarians alternate between the two forms at different stages of their life cycle

Hydrozoans The most primitive cnidarians May be marine or freshwater Life cycle includes both medusa and polyp Includes:  Hydra: individual polyp  Portuguese man-of-war: colony of both medusas and polyps

Scyphozoans Often called true jellyfish Active predators in the medusa stage Many also have a small polyp stage Relatives include:  Box jellies: with a cube shaped medusa  Comb jellies: tentacles have no nematocysts, only a sticky substance to trap prey

Anthozoans The largest class of Cnidarians Exist only as polyps Sea anemones and corals Many live in a symbiotic relationship with algae