Coelenterata is an obsolete yet common term encompassing two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies). The taxon name comes from the Greek &quot;koilos&quot; (&quot;hollow&quot;), referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, external and internal.
Cnidarians are the simplest organisms that have attained a tissue level of organization. All members of this phylum possess stinging structures called nematocysts housed in specialized cells (cnidocytes). Which are considered to be the most primitive nervous elements in the animal kingdom They consist of little more than an outer covering (the epidermis) and an inner gastrodermis with a non-cellular mesoglea sandwiched between the two (Fig 1). Mucus-secreting cells cover the animal with a protective slime. A single opening serves as the entrance to the gastrovascular cavity, within which prey is digested. Lacking an anus, any undigested material is ejected through the mouth. With most of the living cells in direct contact with the water, specialized excretory, circulatory, and respiratory are not needed. Nonetheless, the nervous and sensory systems are reasonably well-developed and many cnidarians are capable of quite complex behavior.
Hydrostatic skeleton: fluid filled: moves like a water balloon. The gastrovascular cavity is where digestion occurs, food enters the mouth and wastes are excreted from the same opening, out of the gastrovascular cavity.
made of a fluid-filled capsule holding a coiled, hollow tube is the nematocyst All members of this phylum possess stinging structures called nematocysts housed in specialized cells (cnidocytes). Cnidocytes are especially concentrated on the tentacles, but are also found scattered over the epidermis and gastrodermis. In some species the poison contained in the barbs of the nematocysts is powerful enough to harm humans (most notably, those of the Portuguese man-of-war, Physalia). Most, however, don't penetrate the skin or if they do they cause only slight irritation.
Those species that alternate between the two body plans are said to have alternation of generations. Polyps usually reproduce through asexual means (budding and fragmentation) while medusae reproduce sexually. Fertilization is external. Sexual reproduction in medusa is dioecious: two separate houses (one male and one female) – monoecious: both sexes in one house - This shows the life cycle and structure of a jellyfish called aurelia.
Anthozoa: soft corals, sea fans, anemones. Scyphozoa: Jellyfish Hydrozoa: Two types, hydroids (the hydra) and siphonophores (Portuguese man-of-war).
Class Hydrozoa: Usually colonial in form, with asexual polyp and sexual medusa stage. Freshwater are likely to have no medusa stage and some marine hydroids do not have free medusae. Hydra is the least common type, but most often used in the classroom. It is freshwater, and solitary with 16 species occurring in North America. Syphonophora are free-swimming or floating colonies. Unlike the hydroids, the medusae are never released from the stem. When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some hydras. Swellings in the body wall develop into either a simple ovary or testes. The testes release free-swimming gametes into the water, and these can fertilise the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating, and, as the adult dies, these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into miniature adults. Hydras are hermaphrodites and may produce both testes and an ovary at the same time.
Feeding polyps called gastrozoide
Class Scyphozoa: Includes most of the larger jellyfishes or “cup animals”. May attain a bell diameter of 2 meters and the tentacles can reach 60-70 meters long, but most are 2-40 cm in diameter. Colorless to bright orange and pink hues.
Class Anthozoa: literally means “flower animal”. Polyps with a flowerlike appearance and no medusa stage. All are marine and found in deep and shallow water, in polar and tropical water. May be colonial or solitary. Many are supported by skeletons. Reef-building corals can be described as miniature sea anemones that live in calcareous cups they have secreted. They can retract into the safety of their cup when not feeding. They take dissolved calcium and carbonate ions from seawater and precipitate it as limestone. They live on top of the layer close to the surface of the ocean. They have unicellular algae (which use photosynthesis) in their tissues that helps provide energy and nutrients. zooxanthellae
These are all very colorful. Sea anemones are larger and heavier polyp. It is very common for Cnidarians to have mutualistic relationships. The two hermit crabs, the clown fish and the anemone and coral with algae.
Comb jellies: contains fewer than 100 species. All are marine and occur in all seas, especially warm waters. Bilateral symmetry rather than radial symmetry of true jellies. Ctenophores (Greek for &quot;comb-bearers&quot;) have eight &quot;comb rows&quot; of fused cilia arranged along the sides of the animal One of the most delightful characteristics of ctenophores is the light-scattering produced by beating of the eight rows of locomotory cilia, which appears as a changing rainbow of colors running down the comb rows. Many people assume that they are seeing bioluminescence when they see this rainbow-effect, but really this is simple light diffraction or scattering of light by the moving cilia. Most (but not all) ctenophores are also bioluminescent, but that light (usually blue or green) can only be seen in darkness. External fertilizing hermaphrodites. This ctenophore managed to out-compete the native planktonic fishes for food, mostly by eating nearly all of the zooplankton in the water before the fish eggs hatched, so there was little left for the native fish larvae. The adult fish remaining in the Black Sea system were also in poor condition as a result of having to compete with the more-successfully-fishing ctenophores for food. Millions of tons of fish in the Black Sea were basically replaced by millions of tons of (inedible) ctenophores.