The members of the Kingdom Animalia are distributed into phyla (singular, phylum), which is equivalent to divisions of the Kingdom Plantae. All animals are heterotrophs. Many members of the Cnidaria have a two part life cycle that includes a sessile (attached) polyp that reproduces asexually and a swimming medusa that reproduces sexually; everything you see is diploid, except eggs and sperm. The free swimming stage distributes the organism away from its parent, a phenomenon called dispersal, and thus the species can conquer the world.

The Phylum Cnidaria contains about 11,000 species that include jelly fishes, hydrozoans, and corals and sea anemones. Most are marine. The Cnidaria are clearly distinct from other animals because they have tentacles with stinging cells called cnidoblasts. Each cnidoblast has a surface trigger and contains a nematocyst that, when fired, harpoons or entangles the prey or attacker and poisons it. The body of a cnidarian is radially symmetrical like a barrel or a wheel. The organism has two cell layers, ectoderm and endoderm or gastroderm (so-called because it lines the digestive sac); nerve cells thread through both of these tissue layers. The mostly non-cellular layer between ectoderm and endoderm is called mesoglea (middle jelly); it was secreted by the cells.

The mouth is central and is surrounded by tentacles. It leads into the gastrovascular cavity. This two-part word refers to the two chief functions of the cavity, digestion and circulation. There are no blood vessels to distribute nutrients from digestion, and the gastrovascular cavity extends out into each of the tentacles. Gas exchange occurs between cells and the surrounding environment; there are no special respiratory structures.

Genus Hydra: Examine the live specimens of Hydra fuscus (brown hydra) and Hydra viridens (green hydra has symbiotic algae). You can see the basal attachment disc, the body, and the tentacles with cnidoblasts surrounding the mouth. Such an attached life stage is called a polyp. Hydra can glide slowly on its base or somersault by bending and placing the tentacles on the substratum. Slides:

Genus Obelia: This genus is a hydrozoan that exhibits alternation of generations. Slides:

Genus Physalia: This colonial hydrozoan consists of a gas-filled float and a colony of feeding polyps that hang deep into the sea. It is commonly called Portugese-man-o-war.

Jellyfish exist primarily in the medusa phase. The mesoglea is quite thick. Aurelia is an example preserved in bioplastic; note the bell shape with a rim of tentacles, central mouth with four oral arms used to capture prey, and four gonads. Radial canals extend from the gastrovascular cavity to a ring canal in the margin of the bell. Sexes are separate and fertilization is internal. Slide:

The planula settles down and metamorphoses into a polyp with tentacles and a mouth, a stage called a scyphistoma; transverse budding produces new medusae, and this stage is termed a strobila. The planula, a small mass of cells that are covered with cilia for swimming, has been suggested as the kind of form that may be ancestral to flatworms and higher invertebrates.

Corals and sea anemones are marine organisms that live attached to rocks or other hard substratum and feed on small fish and invertebrates. Familiar corals are colonial, reef forming organisms; the epidermis of these polyps secretes a rock-like protective skeletal support of calcium carbonate. When polyps build on top of the skeletons of others, a reef may ultimately be formed. Anemones are soft bodied, solitary polyps, such as the preserved specimens of Metridium. When you cut one open, you will find that the wall projects into the gastrovascular cavity as a series of radial partitions. These greatly increase the internal surface area for absorption of nutrients, and the animals can be much bigger than Hydra. The external skeleton of corals also provides support for such partitions.

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