Figure 1. Cnidarian life cycles. Anthozoa (dashed circle on left) consists exclusively of polyps, while Medusozoa (dashed circle on right) consists of both medusas and polyps. The sexual phase, in which eggs and spermatozoa are produced, occurs in anthozoan polyps and medusozoan medusas or reduced medusoids attached to polyps. Polyps alone are produced by sexual reproduction (single arrow). Medusas are produced by the asexual reproduction of polyps (double arrow).

Figure 2. Light micrograph of section through wall of young sea anemone Bunodactis verrucosa at the origin of a mesentery. Two cellular layers are separated by a dense mesoglea.

Figure 3. Phase-contrast micrograph of macerated Hydra viridis cells spread on a haemocytometer grid. Large endodermal cells containing refractive endosymbiotic algae have basal muscular extensions. Smaller cnidoblasts in nests of 2–8 cells contain differentiating cnidocysts.

Figure 4. Cnidocysts (penetrating nematocysts known as euryteles) from the jellyfish Cyanea capillata. (a) Encapsulated cynidocysts and (b) evacuated cnidocysts with everted tubules are differential interference contrast micrographs, and (c) is a scanning electron micrograph. The cnidocyst capsules measure 8 m×14 m, and the discharged tubule is 310 m long. Micrographs by Professor Carina Östman, (Uppsala University, Sweden).