Identify the common attributes of phylum Cnidaria

Cnidarians stand for a more complicated level of organization than Porifera. They possess outer and inner organization layers the sandwich a noncellular mesoglea. Cnidarians possess a well-formed cradle system and carry the end extracellular digestion. The cnidocyte is a devoted cell for transporting toxins come prey as well as warning off predators. Cnidarians have actually separate sexes and have a lifecycle that requires morphologically distinct forms. This animals also show two distinctive morphological forms—medusoid and also polypoid—at various stages in their lifecycle.

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Learning Objectives

Identify common structural and also organization features of the phylum CnidariaIdentify the functions of animals classified in class AnthozoaIdentify the attributes of animals classified in class ScyphozoaIdentify the features of animals classified in class CubozoaIdentify the features of pets classified in course Hydrozoa

Characteristics that Phylum Cnidaria

Phylum Cnidaria includes pets that display radial or biradial symmetry and are diploblastic, the is, they construct from 2 embryonic layers. Virtually all (about 99 percent) cnidarians are naval species.

Cnidarians contain specialized cells well-known as cnidocytes (“stinging cells”) containing oribelles called nematocysts (stingers). This cells are present around the mouth and also tentacles, and serve come immobilize prey v toxins had within the cells. Nematocysts contain coiled subject that may bear barbs. The outer wall surface of the cell has hairlike projections dubbed cnidocils, which space sensitive come touch. When touched, the cells are recognized to fire coiled subject that deserve to either penetrate the flesh of the prey or predators the cnidarians (see Figure 1) or ensnare it. These coiled threads release toxins right into the target and also can regularly immobilize prey or scare far predators.


Figure 1. Animals from the phylum Cnidaria have actually stinging cells called cnidocytes. Cnidocytes contain big organelles referred to as (a) nematocysts that save a coiled thread and also barb. As soon as hairlike projections ~ above the cell surface space touched, (b) the thread, barb, and a toxin space fired from the organelle.


View this video animation showing two anemones involved in a battle.


Figure 2. Cnidarians have two distinctive body plans, the medusa (a) and also the polyp (b). Every cnidarians have actually two membrane layers, v a jelly-like mesoglea in between them.


Animals in this phylum display screen two distinct morphological body plans: polyp or “stalk” and medusa or “bell” (Figure 2). An instance of the polyp form is Hydra spp.; probably the most well-known medusoid pets are the jellies (jellyfish). Polyp forms are sessile together adults, v a single opening come the digestive mechanism (the mouth) encountering up through tentacles neighboring it. Medusa creates are motile, through the mouth and also tentacles hanging under from one umbrella-shaped bell.

Some cnidarians are polymorphic, that is, they have two body plans during their life cycle. An instance is the early american hydroid dubbed an Obelia. The sessile polyp form has, in fact, two types of polyps, displayed in Figure 3. The very first is the gastrozooid, which is adapted for catching prey and also feeding; the other type of polyp is the gonozooid, adapted for the asexual budding of medusa. As soon as the reproductive buds mature, they break off and also become free-swimming medusa, which space either male or mrs (dioecious). The male medusa renders sperm, vice versa, the female medusa provides eggs. After fertilization, the zygote creates into a blastula, which establishes into a planula larva. The larva is free swimming for a while, however eventually attaches and also a brand-new colonial reproductive polyp is formed.


Figure 3. The sessile type of Obelia geniculate has two varieties of polyps: gastrozooids, i m sorry are adapted for capturing prey, and gonozooids, i beg your pardon bud to develop medusae asexually.


Click below to monitor the life cycle of the Obelia.

All cnidarians show the presence of 2 membrane class in the body the are obtained from the endoderm and ectoderm that the embryo. The outer layer (from ectoderm) is referred to as the epidermis and present the external of the animal, conversely, the inner class (from endoderm) is dubbed the gastrodermis and present the cradle cavity. In between these 2 membrane layers is a non-living, jelly-like mesoglea connective layer. In regards to cellular complexity, cnidarians show the presence of differentiated cell species in each tissue layer, such together nerve cells, contractile epithelial cells, enzyme-secreting cells, and also nutrient-absorbing cells, and the existence of intercellular connections. However, the breakthrough of guts or body organ systems is not advanced in this phylum.

The nervous system is primitive, through nerve cells scattered across the body. This nerve net may display the existence of teams of cells in the form of nerve plexi (singular plexus) or nerve cords. The nerve cells present mixed characteristics of motor and sensory neurons. The primary signaling molecule in this primitive nervous systems are chemical peptides, which perform both excitatory and also inhibitory functions. Despite the simplicity that the worried system, it collaborates the activity of tentacles, the illustration of captured prey to the mouth, the digestion of food, and the expulsion that waste.

The cnidarians perform extracellular digestion in which the food is taken right into the gastrovascular cavity, enzymes space secreted right into the cavity, and also the cell lining the cavity absorb nutrients. The gastrovascular cavity has only one opening that serves together both a mouth and an anus, which is termed one incomplete cradle system. Cnidarian cell exchange oxygen and also carbon dioxide through diffusion between cells in the epidermis with water in the environment, and between cell in the gastrodermis v water in the gastrovascular cavity. The absence of a circulatory device to move dissolved gases boundaries the thickness that the body wall surface and necessitates a non-living mesoglea between the layers. Over there is no excretory system or organs, and also nitrogenous wastes simply diffuse indigenous the cells into the water external the pet or in the gastrovascular cavity. Over there is additionally no circulatory system, so nutrient must move from the cells that absorb them in the lining the the gastrovascular cavity through the mesoglea to other cells.

The phylum Cnidaria contains around 10,000 described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and also Hydrozoa. The anthozoans, the sea anemones and also corals, space all sessile species, vice versa, the scyphozoans (jellyfish) and also cubozoans (box jellies) room swimming forms. The hydrozoans save sessile forms and also swimming colonial forms prefer the Portuguese man O’ War.

Classes in the Phylum Cnidaria

Class Anthozoa

The course Anthozoa includes all cnidarians the exhibit a polyp body plan only; in other words, there is no medusa phase within your life cycle. Examples include sea anemones (Figure 4), sea pens, and corals, with an estimated number of 6,100 defined species. Sea anemones space usually brightly colored and can achieve a size of 1.8 come 10 centimeter in diameter. These animals are generally cylindrical in shape and also are attached come a substrate. A mouth opening is surrounded by tentacles bearing cnidocytes.


Figure 4. The sea anemone is presented (a) photographed and also (b) in a diagram showing its morphology. (credit a: modification of work-related by “Dancing with Ghosts”/Flickr; credit transaction b: modification of work by NOAA)


The mouth the a sea anemone is surrounding by tentacles that bear cnidocytes. The slit-like mouth opening and pharynx are lined through a groove dubbed a siphonophore. The pharynx is the muscular part of the digestive system that offer to ingest as well as egest food, and may expand for as much as two-thirds the size of the body before opening into the gastrovascular cavity. This cavity is split into number of chambers by longitudinal septa called mesenteries. Every mesentery is composed of one ectodermal and one endodermal cell layer with the mesoglea sandwiched in between. Mesenteries execute not division the gastrovascular cavity completely, and the smaller cavities coalesce in ~ the pharyngeal opening. The adaptive advantage of the mesenteries appears to be an increase in surface ar area because that absorption that nutrients and gas exchange.

Sea anemones feed on tiny fish and shrimp, normally by immobilizing your prey making use of the cnidocytes. Some sea anemones create a mutualistic relationship with hermit crabs by attaching to the crab’s shell. In this relationship, the anemone gets food particles from prey captured by the crab, and the crab is safeguarded from the predators through the stinging cell of the anemone. Anemone fish, or clownfish, room able come live in the anemone due to the fact that they room immune to the toxins consisted of within the nematocysts.

Anthozoans stay polypoid throughout your lives and can reproduce asexually by budding or fragmentation, or sexually by creating gametes. Both gametes are developed by the polyp, which can fuse to provide rise come a free-swimming planula larva. The larva settles on a perfect substratum and develops right into a sessile polyp.

Class Scyphozoa

Class Scyphozoa consists of all the jellies and is exclusively a marine class of animals with around 200 well-known species. The defining characteristic the this course is the the medusa is the prominent stage in the life cycle, although there is a polyp phase present. Members the this species range indigenous 2 to 40 cm in length yet the biggest scyphozoan species, Cyanea capillata, deserve to 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 also (b) in a diagram depicting its morphology. (credit a: modification of job-related by “Jimg944″/Flickr; credit b: modification of job-related by Mariana Ruiz Villareal)


In the jellyfish, a mouth opening is present on the underside the the animal, surrounded by tentacles bearing nematocysts. Scyphozoans live many of their life cycle together free-swimming, solitary carnivores. The mouth leader to the gastrovascular cavity, which may be sectioned into 4 interconnected sacs, referred to as diverticuli. In part species, the cradle system might be further branched right into radial canals. Choose the septa in anthozoans, the branched gastrovascular cell serve 2 functions: to rise the surface ar area for nutrient absorption and diffusion; thus, much more cells are in direct contact with the nutrient in the gastrovascular cavity.

In scyphozoans, nerve cells are scattered everywhere the body. Neurons may even be existing in clusters dubbed rhopalia. These pets possess a ring of muscle lining the dome that the body, which offers the contractile pressure required to swim with water. Scyphozoans room dioecious animals, the is, the sexes room separate. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are developed by outside fertilization; they work out on a substratum in a polypoid type known as scyphistoma. These develops may produce extr polyps by budding or may transform right into the medusoid form. The life bicycle (Figure 6) that these animals can be explained as polymorphic, because they exhibition both a medusal and polypoid body setup at some suggest in their life cycle.


Figure 6. The lifecycle of a jellyfish has two stages: the medusa stage and also the polyp stage. The polyp reproduces asexually through budding, and the medusa reproduces sexually. (credit “medusa”: modification of occupational by Francesco Crippa)


Identify the life cycle stages of jellies utilizing this video computer animation quiz from the brand-new England Aquarium.

Class Cubozoa

This class includes jellies that have a box-shaped medusa, or a bell that is square in cross-section; hence, space colloquially recognized as “box jellyfish.” These varieties may achieve sizes the 15–25 cm. Cubozoans display overall morphological and anatomical characteristics that are comparable to those that the scyphozoans. A prominent difference between the 2 classes is the arrangement of tentacles. This is the many venomous group of all the cnidarians (Figure 7).

The cubozoans save on computer muscular pads called pedalia at the corners the the square bell canopy, through one or much more tentacles enclosed to every pedalium. These animals are more classified into orders based upon the existence of single or multiple tentacles every pedalium. In some cases, the digestive device may extend into the pedalia. Nematocysts may be arranged in a spiral configuration follow me the tentacles; this setup helps to successfully subdue and also capture prey. Cubozoans exist in a polypoid type that establishes from a planula larva. These polyps show restricted mobility along the substratum and, prefer scyphozoans, may bud to type more polyps come colonize a habitat. Polyp creates then transform into the medusoid forms.


Figure 7. The (a) tiny cubazoan jelly Malo kingi is thimble shame and, like all cubozoan jellies, (b) has 4 muscular pedalia come which the tentacles attach. M. Kingi is one of two species of jellies recognized to cause Irukandji syndrome, a condition defined by excruciating muscle pain, vomiting, boosted heart rate, and psychological symptoms. Two civilization in Australia, where Irukandji jellies are most commonly found, are believed to have passed away from Irukandji stings. (c) A authorize on a coast in northern Australia advises swimmers of the danger. (credit c: alteration of job-related by Peter Shanks)


Class Hydrozoa

Hydrozoa includes almost 3,200 species; many are marine, although some freshwater types are recognized (Figure 8). Pets in this course are polymorphs, and also most exhibit both polypoid and also medusoid forms in their lifecycle, return this is variable.

The polyp kind in these pets often shows a cylindrical morphology with a main gastrovascular cavity inside wall by the gastrodermis. The gastrodermis and also epidermis have a basic layer the mesoglea sandwiched in between them. A mouth opening, surrounding by tentacles, is current at the oral finish of the animal. Countless hydrozoans type colonies that are composed of a branched nest of devoted polyps the share a gastrovascular cavity, such as in the colonial hydroid Obelia. Nests may additionally be free-floating and contain medusoid and also polypoid people in the colony as in Physalia (the Portuguese man O’ War) or Velella (By-the-wind sailor). Also other species are solitary polyps (Hydra) or solitary medusae (Gonionemus). The true characteristic common by all of these diverse species is that their gonads for sex-related reproduction are derived from epidermal tissue, conversely, in all other cnidarians they are acquired from gastrodermal tissue.


Figure 8. (a) Obelia, (b) Physalia physalis, well-known as the Portuguese guy O’ War, (c) Velella bae, and also (d) Hydra have various body shapes but all belong come the family members Hydrozoa. (credit b: change of work by NOAA; scale-bar data native Matt Russell)


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