Flowers of the sea, the hydroids, anemones, corals and jelly-fishes, all bearing whorls of stinging tentacles around their mouths, are stubbed with tiny stinging nematocysts. Long coiled threads of toxic proteins uncoil rapidly and stab into an unsuspecting victim delivering a severe and immobilizing sting. Most nematocysts, however are too small to penetrate human skin. In the aquarium, fish and other mobile invertebrates seem to be aware of the danger and keep their distance. Jellyfish should not be kept in the aquarium. They and ctenophores should be kept in separate containers, such as large plastic garbage cans filled with sea water, without aeration or agitation to prolong their life. Hydroids, anemones, hard and soft corals should be fed every few days with freshly hatched brine shrimp, or clam juice pipetted directly over the polyps.

The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions.

Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella.

Polyps, in contrast, are usually sessile. They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense.

Reproduction in polyps is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming.

If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death.

The diversity of living organisms on earth is truly astounding, almost overwhelming. Humans have come up with ways of organizing, or classifying, biological diversity throughout human history. Organisms can be classified according to any number of criteria, including overall similarities, colors, ecological functions, etc. However, it is generally agreed that the most useful way for scientists to organize biological diversity is to group organisms according to shared evolutionary history. This way the grouping not only results in an organized classification, it also contains and conveys information about our understanding of the evolutionary history of these groups.

Although our understanding of evolutionary relationships among organisms has greatly improved in the last century, it is by no means complete. Relationships among organisms, and groups of organisms, continues to be revised as new data becomes available. The rate of such revisions has increased in recent years primarily as a result of the huge amount of new molecular data (such as DNA sequences) that has been brought to bear on tests of evolutionary relationships. This means that nearly all taxonomies (systems of nomenclature) based on evolutionary relationships among organisms are being revised, sometimes radically so. Traditional ideas about how organisms are related, and in which groups they belong, often prove inaccurate.

Traditional, biological classification schemes included the idea of “ranks”, such as species, genus, family, order, class, etc. In this system (the Linnean system), for example, there is a Class Reptilia and a Class Aves. However, the bulk of evidence supports, and the majority of scientists now agree that, the group Aves belongs within the larger group Reptilia (birds share a most recent common ancestor with crocodiles, which are generally included in the Class Reptilia). Within a traditional, Linnean system of classification this means that either the Class Aves is demoted to something below a class, or that a class (Aves) exists within another class (Reptilia). Problems such as this have prompted many scientists to propose that a system of naming and classification of biological diversity be rank-free. Classification systems then only indicate the hierarchical structure of groups according to the current understanding of their evolutionary history, leaving out rank labels.

The Animal Diversity Web prefers a rank-free classification, and uses such a format on our classification pages. However, because rank labels are still used extensively in biological education, we retain rank labels at certain levels, including class, order, and family. In doing so we hope to provide an opportunity for educators to discuss the issues of ranks, classification systems, and our understanding of the evolutionary history of organisms in their classrooms. Please consider the rank labels retained on the ADW as convenient place-markers to indicate the correspondence of current, phylogenetic classification schemes with traditional classification systems.

Classification – a system of naming objects or entities by common characteristics. In a biological sense, classification is the systematic grouping of organisms based on structural or functional similarities or evolutionary history. A process of establishing, defining, and ranking taxa within hierarchical series of groups.

Taxonomy – the classification of organisms into a system that indicates natural relationships (evolutionary relationships); the theory and practice of describing, naming, and classifying organisms.

Systematics – the systematic classification of organisms and the evolutionary relationships among them; taxonomy.

Phylogeny – the evolutionary history of a group or lineage.

Nomenclature – the system of scientific names applied to taxa (groups of organisms).

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