Medusa left and polyp right  Oral end of actinodiscus polyp, with close-up of the mouth Most adult cnidarians appear as either swimming medusae or sessile polypsand many hydrozoan species are known to alternate between the two forms.
Unlike the closely related corals, these organisms do not have a skeleton. Click the link for more information. The skeleton itself is Cnidaria true jellyfish called coral. The body of a jellyfish is shaped like a bell or umbrella, with a clear, jellylike material filling most of the space between Cnidarians are radially symmetrical see symmetry, biological symmetry, biological, similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed.
Symmetry in body shapes is related to the lifestyles of organisms. The mouth, located at the center of one end of the body, opens into a gastrovascular cavity, which is used for digestion and distribution of food; an anus is lacking.
Cnidarians are further characterized by having a body wall composed of three layers: Tentacles encircle the mouth and are used in part for food capture. Specialized stinging structures, called nematocysts, are a characteristic of the phylum and are borne in the tentacles and often in other body parts.
These contain a coiled fiber that can be extruded suddenly. Some nematocysts contain toxic substances and are defense mechanisms, while others are adhesive, helping to anchor the animal or to entangle prey.
Two body forms and two lifestyles are characteristic of the Cnidaria see polyp and medusa polyp and medusa, names for the two body forms, one nonmotile and one typically free swimming, found in the aquatic invertebrate phylum Cnidaria the coelenterates. Some animals of this group are always polyps, some are always medusae, and some exhibit both a polyp and a medusa The sessile hydroid, or polyp, form is more or less cylindrical, attached to its substratum at its aboral opposite the mouth end, with the mouth and surrounding tentacles at the upper, oral, free end.
Colonies of hydroids comprise several different types of individuals: The motile jellyfish, or medusoid form, is flattened, with the tentacles usually located at the body margin. With few exceptions, the cnidarians are marine.
There are over 9, known living species; fossil records of cnidarians date back to the Ordovician era. Cnidarians are carnivorous, the major part of their diet consisting of crustaceans. Animals in this phylum have no specialized excretory or respiratory organs but do have a nervous system.
Both sexual and asexual reproduction occur. There are three classes of cnidarians. Class Hydrozoa The Hydrozoa include solitary or colonial cnidarians, which have a noncellular mesoglea, lack tentacles within the gastrovascular cavity, and have no gullet. As a rule, the hydroid stage predominates in the life cycle, although in some the jellyfish stage is larger.
The order Hydroida includes the many small, colonial hydroids so often seen clinging to wharves and submerged objects along the seacoasts everywhere, economically important because they foul surfaces.
The order also includes solitary hydroids, some reaching several inches in height. One, in the genus Branchiocerianthus, is said to reach 8 or 9 ft — The common freshwater genus Hydra also belongs to this order, as does the freshwater jellyfish, genus Craspedacusta, and the commonly studied hydroid jellyfish, genus Gonionemus.
There are also pelagic hydroid colonies, unusual in having one very large hydroid member, which lives with its mouth downward and its aboral surface upward, like a jellyfish. The aboral end is equipped with a projecting sail. Velella, the purple sailor, is an example.
The order Milleporina includes colonial organisms that form a massive, porous exoskeleton, somewhat resembling corals. They are sometimes abundant in tropical seas and may contribute to coral reef formation. The order Siphonophora includes often large, floating colonies made up of members of varying form and function.
Typical is Physalia, the Portuguese man-of-war.Cnidaria is a large phylum composed of some of the most beautiful of all the salt and freshwater organisms: the true jellyfish, box jellyfish, coral and sea anemones, and hydra. Although Cnidaria is an incredibly diverse group of animals, there are several traits that link them together.
Jellyfish are found all over the world, from surface waters to the deep sea. Scyphozoans (the "true jellyfish") are exclusively marine, but some hydrozoans with a similar appearance live in pfmlures.com, often colorful, jellyfish are common in coastal zones pfmlures.comlum: Medusozoa.
Scyphozoans include most of the jellyfish familiar to beach-goers; other similar organisms are classified in the Hydrozoa and Cubozoa, two other groups of cnidarians. True jellyfish are graceful, and sometimes deadly creatures. Jellyfish and Other Cnidarians Cnidaria is a phylum containing some 11, species of relatively simple invertebrate animals found exclusively in aquatic, mostly marine, environments.
Cniderians include corals, sea anemones, jellyfish, sea pens, sea pansies, and sea wasps, and tiny freshwater hydra.
Predators of Cnidaria include sea slugs, sea stars (for example, the Crown of Thorn which can devastate coral reefs), nudibranchs, fish including butterfly fish and parrot fish, which eat corals and marine turtles and sunfish, which eat jellyfish. Scyphozoans include most of the jellyfish familiar to beach-goers; other similar organisms are classified in the Hydrozoa and Cubozoa, two other groups of cnidarians.
True jellyfish are graceful, and sometimes deadly creatures.