Tissue Layers Evolve

Tissue Layers Evolve

Cnidaria and comb jellies have two tissue layers derived from the germ layers ectoderm and endoderm. They are radially symmetrical.
Cnidaria have a sac body plan. They exist as either polyps or medusae, or they can alternate between the two. Hydras and
relatives—sea anemones and corals—are polyps; in jellyfishes the medusan stage is dominant. In Hydra and other cnidaria, an outer epidermis is separated from an inner gastrodermis by mesoglea.
They possess tentacles to capture prey and nematocysts to stun it. A nerve net coordinates movements. Digestion of prey begins in the gastrovascular cavity and is finished within gastrodermal cells.