In the jellyfish, a mouth opening, surrounded by tentacles bearing nematocysts, is present on the underside of the animal. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may be further branched into radial canals.
Like the septa in anthozoans, the branched gastrovascular cells serves to increase the surface area for nutrient absorption and diffusion; thus, more cells are in direct contact with the nutrients in the gastrovascular cavity. In scyphozoans, nerve cells are scattered over the entire body. Neurons may even be present in clusters called rhopalia.
These animals possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water. Scyphozoans are dioecious animals, having separate sexes. The gonads are formed from the gastrodermis with gametes expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. These forms may produce additional polyps by budding or may transform into the medusoid form.
The life cycle of these animals can be described as polymorphic because they exhibit both a medusal and polypoid body plan at some point. Lifecycle of a jellyfish : The lifecycle of a jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding,while the medusa reproduces sexually. Cubozoans live as box-shaped medusae while Hydrozoans are true polymorphs and can be found as colonial or solitary organisms.
Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians. Cubozoans : The a tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, b has four muscular pedalia to which the tentacles attach.
Two people in Australia, where Irukandji jellies are most-commonly found, are believed to have died from Irukandji stings. The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium.
In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum.
As with scyphozoans, they may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms. Hydrozoa includes nearly 3, species; most are marine, although some freshwater species are known.
Animals in this class are polymorphs: most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable. The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal.
Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia. Other species are solitary polyps Hydra or solitary medusae Gonionemus. The creatures in this phylum are radially symmetrical. This means that the parts of the body extend outward from the center like the spokes on a bicycle wheel.
A common example of radial symmetry is the sea star a member of the Echinoderm phylum or the anemone, a Cnidarian seen below. The Cnidarians include the hydroids, jellyfish, anemones, and corals. All Cnidarians have tentacles with stinging cells in their tips which are used to capture and subdue prey. In fact, the phylum name "Cnidarian" literally means "stinging creature.
The nematocyst is a coiled thread-like stinger. When the nematocyst is called upon to fire, the thread is uncoiled, and springs straight. The harpoon-like thread punctures through the cnidocyte wall and into the prey.
All Cnidaria can reproduce asexually by various means, in addition to regenerating if their bodies are divided into segments or are attacked by predators. Some produce buds, others divide down the middle, others do both. Cnidarians range in size from Hydra, 5—20 millimetres to the Lion's mane jellyfish, which may exceed 2 metres in diameter and 75 metres in length. Thanks to I would sincerely like to thank the many members of the Flickr community who have given me permission to use their wonderful images for this unit.
Their contributions really make this unit come alive! Next: Anthozoa Cnidaria Cnidaria is a phylum containing over 9, species found only in aquatic and mostly marine environments.
Diversity in Cnidara. Body form of Cnidaria. How a nematocyst works. Life cycle of a jellyfish. Porifera and Cnidaria. Cnidarians Informational Video 5.
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.
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