What Is The Difference Between A Polymorphic Life Cycle And The Typical Life Cycle Of Other Animals

Phylum Cnidaria

Cnidarians are diploblastic, have organized tissue, undergo extracellular digestion, and use cnidocytes for protection and to capture casualty.

Learning Objectives

Describe the key anatomy of a Cnidarian

Primal Takeaways

Key Points

• Cnidarians have two singled-out morphological body plans known as polyp, which are sessile as adults, and medusa, which are mobile; some species showroom both body plans in their lifecycle.
• All cnidarians have 2 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 pause downwardly the food particles and cells lining the gastrovascular crenel absorb the nutrients.
• Cnidarians have an incomplete digestive organization with merely one opening; the gastrovascular crenel serves every bit both a mouth and an anus.
• The nervous arrangement 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 brand up the iv different classes of Cnidarians.

Key Terms

• diploblastic: having 2 embryonic germ layers (the ectoderm and the endoderm)
• cnidocyte: a sheathing, in certain cnidarians, containing a spinous, threadlike tube that delivers a paralyzing sting

Introduction to Phylum Cnidaria

Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic: they develop from two embryonic layers. Nearly all (nigh 99 percent) cnidarians are marine species.

Cnidarians contain specialized cells known every bit cnidocytes ("stinging cells"), which comprise organelles called nematocysts (stingers). These cells are nowadays around the mouth and tentacles, serving to immobilize casualty with toxins contained within the cells. Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch on. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the casualty or predators of cnidarians, or ensnare it. These coiled threads release toxins into the target that can often immobilize prey or scare abroad predators ().

Cnidocytes: Animals from the phylum Cnidaria take stinging cells chosen cnidocytes. Cnidocytes comprise big organelles called (a) nematocysts that shop a coiled thread and barb. When hairlike projections on the cell surface are touched, (b) the thread, barb, and a toxin are fired from the organelle.

Animals in this phylum brandish two distinct morphological trunk plans: polyp or "stalk" and medusa or "bong". An instance of the polyp course is Hydra spp.; maybe the most well-known medusoid animals are the jellies (jellyfish). Polyp forms are sessile as adults, with a single opening to the digestive system (the oral cavity) facing up with tentacles surrounding it. Medusa forms are motile, with the oral cavity and tentacles hanging downwards from an umbrella-shaped bell.

Cnidarian morphology: Cnidarians have ii distinct body plans, the medusa (a) and the polyp (b). All cnidarians accept two membrane layers, with a jelly-similar mesoglea betwixt them.

Some cnidarians are polymorphic, having two body plans during their life cycle. An example is the colonial hydroid chosen an Obelia. The sessile polyp form has, in fact, two types of polyps. The beginning is the gastrozooid, which is adapted for capturing casualty and feeding; the other type of polyp is the gonozooid, adapted for the asexual budding of medusa. When the reproductive buds mature, they interruption off and become costless-swimming medusa, which are either male or female (dioecious). The male medusa makes sperm, whereas the female medusa makes eggs. Later fertilization, the zygote develops into a blastula and then into a planula larva. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed.

Types of polyps in Obelia : The sessile course of Obelia geniculate has ii types of polyps: gastrozooids, which are adapted for capturing casualty, and gonozooids, which bud to produce medusae asexually.

All cnidarians prove the presence of 2 membrane layers in the body that are derived from the endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive cavity. Between these two membrane layers is a not-living, jelly-like mesoglea connective layer. In terms of cellular complication, cnidarians evidence the presence of differentiated prison cell types in each tissue layer: nerve cells, contractile epithelial cells, enzyme-secreting cells, and food-arresting cells, as well every bit the presence of intercellular connections. Withal, the development of organs or organ systems is not advanced in this phylum.

The nervous system is archaic, with nervus cells scattered across the body. This nerve internet may show the presence of groups of cells in the form of nerve plexi (singular: plexus) or nerve cords. The nervus cells show mixed characteristics of motor as well equally sensory neurons. The predominant signaling molecules in these primitive nervous systems are chemic peptides, which perform both excitatory and inhibitory functions. Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the cartoon of captured prey to the oral cavity, the digestion of food, and the expulsion of waste.

The cnidarians perform extracellular digestion in which the food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity blot nutrients. The gastrovascular cavity has only one opening that serves every bit both a oral fissure and an anus; this is termed an incomplete digestive system. Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis with water in the surroundings, and between cells in the gastrodermis with water in the gastrovascular cavity. The lack of a circulatory arrangement to move dissolved gases limits the thickness of the trunk wall, necessitating a non-living mesoglea between the layers. There is no excretory system or organs; nitrogenous wastes simply lengthened from the cells into the h2o outside the animal or in the gastrovascular cavity. There is also no circulatory system, so nutrients must move from the cells that absorb them in the lining of the gastrovascular cavity through the mesoglea to other cells.

The phylum Cnidaria contains nearly 10,000 described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. The anthozoans, the sea anemones and corals, are all sessile species, whereas the scyphozoans (jellyfish) and cubozoans (box jellies) are swimming forms. The hydrozoans contain sessile forms and pond colonial forms like the Portuguese Homo O' War.

Class Anthozoa

Members of the class Anthozoa display only polyp morphology and accept cnidocyte-covered tentacles around their oral fissure opening.

Learning Objectives

Identify the adaptive features of anthozoa

Key Takeaways

Primal Points

• Anthozoans include sea anemones, sea pens, and corals.
• The throat of anthozoans (ingesting as well as egesting food) leads to the gastrovascular cavity, which is divided by mesenteries.
• In Anthozoans, gametes are produced by the polyp; if they fuse, they will give ascent to a free-swimming planula larva, which volition become sessile one time information technology finds an optimal substrate.
• Body of water anemonies and coral are examples of anthozoans that form unique mutualistic relationships with other creature species; both body of water anemonies and coral benefit from food availability provided by their partners.

Key Terms

• mesentery: in invertebrates, information technology describes any tissue that divides the body cavity into partitions
• cnidocyte: a capsule, in certain cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting
• hermatypic: of a coral that is a species that builds coral reefs

Class Anthozoa

The class Anthozoa includes all cnidarians that showroom a polyp body program only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of six,100 described species. Sea anemones are ordinarily brightly colored and can reach a size of one.viii to 10 cm in diameter. These animals are usually cylindrical in shape and are fastened to a substrate.

Anthozoans: The sea anemone (a), like all anthozoans, has only a polyp body plan (b).

The oral fissure of a sea anemone is surrounded past tentacles that behave cnidocytes. They take slit-similar mouth openings and a throat, which is the muscular function of the digestive system that serves to ingest besides as egest nutrient. Information technology may extend for upwardly to 2-thirds the length of the torso earlier opening into the gastrovascular cavity. This cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of one ectodermal and i endodermal cell layer with the mesoglea sandwiched in between. Mesenteries do not divide the gastrovascular crenel completely; the smaller cavities coalesce at the pharyngeal opening. The adaptive do good of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas commutation.

Bounding main anemones feed on small fish and shrimp, usually by immobilizing their prey using the cnidocytes. Some sea anemones establish a mutualistic human relationship with hermit venereal by attaching to the crab's beat. In this relationship, the anemone gets food particles from prey caught by the crab, while the crab is protected from the predators by the stinging cells of the anemone. Anemone fish, or clownfish, are able to live in the anemone since they are immune to the toxins independent within the nematocysts. Some other blazon of anthozoan that forms an of import mutualistic relationship is reef edifice coral. These hermatypic corals rely on a symbiotic relationship with zooxanthellae. The coral gains photosynthetic capability, while the zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the cnidarian host.

Anthozoans remain polypoid throughout their lives. They can reproduce asexually by budding or fragmentation, or sexually past producing gametes. Both gametes are produced past the polyp, which can fuse to give ascension to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp.

Class Scyphozoa

Scyphozoans are free-swimming, polymorphic, dioecious, and carnivorous cnidarians with a prominent medusa morphology.

Learning Objectives

Explain the cardinal features of scyphozoa

Key Takeaways

Primal Points

• Scyphozoans have a ring of muscles that lines the dome of their bodies; these structures provide them with the contractile strength they need to swim through water.
• Scyphozoans take separate sexes and form planula larvae through external fertilization.
• Jellies exhibit the polyp form, known as a scyphistoma, later on their larvae settle on a substrate; these forms will later bud-off and transform into their more prominenent medusa forms.

Central Terms

• dioecious: having the male and female person reproductive organs on split parts (of the same species)
• rhopalia: small sensory structures establish inside Scyphozoa that are characterized by clusters of neurons that tin be used to sense light
• scyphistoma: the polypoid grade of scyphozoans
• nematocyst: a capsule, in sure cnidarians, containing a barbed, threadlike tube that delivers a paralyzing sting

Course Scyphozoa

Class Scyphozoa, an exclusively marine class of animals with almost 200 known species, includes all the jellies. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp phase nowadays. Members of this species range from two to 40 cm in length, only the largest scyphozoan species, Cyanea capillata, tin accomplish a size of 2 one thousand across. Scyphozoans display a characteristic bong-like morphology.

Scyphozoans: For jellyfish (a), and all other scyphozoans, the medusa (b) is the most prominent of the two life stages.

In the jellyfish, a rima oris opening, surrounded past tentacles begetting nematocysts, is present on the underside of the creature. Scyphozoans live most of their life bicycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may exist 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 increment the expanse for nutrient assimilation and improvidence; thus, more cells are in directly contact with the nutrients in the gastrovascular cavity.

In scyphozoans, nervus 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 trunk, which provides the contractile forcefulness 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 showroom 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 past budding,while the medusa reproduces sexually.

Class Cubozoa and Class Hydrozoa

Cubozoans live as box-shaped medusae while Hydrozoans are true polymorphs and can be found every bit colonial or solitary organisms.

Learning Objectives

Distinguish betwixt cubozoa and hydrozoa cnidarians

Key Takeaways

Key Points

• Cubozoans differ from Scyphozoans in their arrangement of tentacles; they are also known for their box-shaped medusa.
• Out of all cnidarians, cubozoans are the most venomous.
• Hydrozoans are polymorphs, existing equally solitary polyps, solitary medusae, or equally colonies.
• Hydrozoans are unique from all other cnidarians in that their gonads are derived from epidermal tissue.

Key Terms

• hydroid: any of many colonial coelenterates that exist mainly every bit a polyp; a hydrozoan

Class Cubozoa

Grade Cubozoa includes jellies that have a box-shaped medusa: a bong that is foursquare in cross-department; hence, they are colloquially known every bit "box jellyfish." These species may achieve sizes of xv–25 cm. Cubozoans display overall morphological and anatomical characteristics that are like to those of the scyphozoans. A prominent difference between the two classes is the system of tentacles. This is the well-nigh 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. Grand. kingi is one of two species of jellies known to crusade Irukandji syndrome, a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological symptoms. Two people in Australia, where Irukandji jellies are most-commonly found, are believed to accept died from Irukandji stings. (c) A sign on a embankment in northern Australia warns swimmers of the danger.

The cubozoans contain muscular pads chosen pedalia at the corners of the square bong awning, with one or more tentacles attached to each pedalium. These animals are farther classified into orders based on the presence of unmarried 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 finer subdue and capture prey. Cubozoans exist in a polypoid class that develops from a planula larva. These polyps show limited mobility forth the substratum. Every bit with scyphozoans, they may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms.

Class Hydrozoa

Hydrozoa includes nearly 3,200 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.

Hydrozoans: (a) Obelia, (b) Physalia physalis, known as the Portuguese Man O' War, (c) Velella bae, and (d) Hydra have different body shapes, but all belong to the family Hydrozoa.

The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular crenel lined past the gastrodermis. The gastrodermis and epidermis accept a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral terminate of the animal. Many hydrozoans course colonies that are equanimous of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia. Colonies may also exist free-floating and contain medusoid and polypoid individuals in the colony as in Physalia (the Portuguese Human being O' War) or Velella (By-the-wind sailor). 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.

Source: https://courses.lumenlearning.com/boundless-biology/chapter/phylum-cnidaria/

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