Cnidarians have a defined top and bottom, and are made up of two layers of tissue that include nerve and muscle cells. With these nerves and muscles, ancestral cnidarians were the first animals to move. The longest animal in the world is a cnidarian!
This process constitutes one of the fastest in biology and is accompanied by a release of toxins that are potentially harmful also for humans.
The long history of research on Hydra as a model organism has been accompanied by the cellular, mechanistic and morphological analysis of its nematocyst repertoire. Although representing one of the most complex organelles of the animal kingdom, the evolutionary origin and molecular map of the nematocyst has remained largely unknown. Class Scyphozoa includes all the jellies and is exclusively a marine class of animals with about known species.
The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. Members of this species range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata , can reach a size of 2 m across.
Scyphozoans display a characteristic bell-like morphology [Figure 5]. Figure 5: A jelly is shown a photographed and b in a diagram illustrating its morphology. In the jellyfish, a mouth opening is present on the underside of the animal, surrounded by tentacles bearing nematocysts. 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 serve two functions: 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 all over the 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, that is, the sexes are separate. The gonads are formed from the gastrodermis and gametes are 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 [Figure 6] of these animals can be described as polymorphic , because they exhibit both a medusal and polypoid body plan at some point in their life cycle.
Figure 6: The lifecycle of a jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually. 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 [Figure 7]. 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 and, like scyphozoans, may bud to form more polyps to colonize a habitat.
Polyp forms then transform into the medusoid forms. Figure 7: 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. 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 true characteristic shared by all these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.
Privacy Policy. Skip to main content. Search for:. Phylum Cnidaria. Phylum Cnidaria Cnidarians are diploblastic, have organized tissue, undergo extracellular digestion, and use cnidocytes for protection and to capture prey. Learning Objectives Describe the fundamental anatomy of a Cnidarian. Key Takeaways Key Points Cnidarians have two distinct morphological body plans known as polyp, which are sessile as adults, and medusa, which are mobile; some species exhibit both body plans in their lifecycle.
All cnidarians have two membrane layers in the body: the epidermis and the gastrodermis; between both layers they have the mesoglea, which is a connective layer. Cnidarians carry out extracellular digestion, where enzymes break down the food particles and cells lining the gastrovascular cavity absorb the nutrients.
Cnidarians have an incomplete digestive system with only one opening; the gastrovascular cavity serves as both a mouth and an anus.
The nervous system of cnidarians, responsible for tentacle movement, drawing of captured prey to the mouth, digestion of food, and expulsion of waste, is composed of nerve cells scattered across the body. Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa make up the four different classes of Cnidarians. Key Terms diploblastic : having two embryonic germ layers the ectoderm and the endoderm cnidocyte : a capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting.
Class Anthozoa Members of the class Anthozoa display only polyp morphology and have cnidocyte-covered tentacles around their mouth opening.
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