Male or female gametes produced by a polyp fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp. The medusa is the prominent stage in the life cycle, although there is a polyp stage in the life cycle of most species.
Most jellies range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata , can reach a size of two meters in diameter. Scyphozoans display a characteristic bell-like morphology Figure. In the sea jelly, a mouth opening is present on the underside of the animal, surrounded by hollow 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 branch further 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, and to support the body of the animal. In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body, with a nerve ring around the edge of the bell. Clusters of sensory organs called rhopalia may be present in pockets in the edge of the bell. Jellies have a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water, as well as to draw in food from the water as they swim.
Scyphozoans have separate sexes. 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. These polyps may bud to form additional polyps or begin immediately to produce medusa buds.
In a few species, the planula larva may develop directly into the medusa. The life cycle Figure of most scyphozoans includes both sexual medusoid and asexual polypoid body forms. However, 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. The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each 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 include the most venomous of all the cnidarians Figure. These animals are unusual in having image-forming eyes, including a cornea, lens, and retina. Because these structures are made from a number of interactive tissues, they can be called true organs. Eyes are located in four clusters between each pair of pedalia.
Each cluster consists of four simple eye spots plus two image-forming eyes oriented in different directions. How images formed by these very complex eyes are processed remains a mystery, since cubozoans have extensive nerve nets but no distinct brain.
Nontheless, the presence of eyes helps the cubozoans to be active and effective hunters of small marine animals like worms, arthropods, and fish. Cubozoans have separate sexes and fertilization occurs inside the female. Planula larvae may develop inside the female or be released, depending on species. Each planula develops into a polyp. These polyps may bud to form more polyps to create a colony; each polyp then transforms into a single medusa.
Class Hydrozoa Hydrozoa is a diverse group that includes nearly 3, species; most are marine, although some freshwater species are known Figure. Most species exhibit both polypoid and medusoid forms in their lifecycles, although the familiar Hydra has only the polyp form. The medusoid form has a muscular veil or velum below the margin of the bell and for this reason is called a hydromedusa.
In contrast, the medusoid form of Scyphozoa lacks a velum and is termed a scyphomedusa. 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 sessile, branched colonies of specialized polyps that share a common, branching gastrovascular cavity coenosarc , such as is found in the colonial hydroid Obelia.
Free-floating colonial species called siphonophores contain both medusoid and polypoid individuals that are specialized for feeding, defense, or reproduction. At first glance, these complex superorganisms appear to be a single organism; but the reality is that even the tentacles are actually composed of zooids laden with nematocysts.
Thus, although it superficially resembles a typical medusozoan jellyfish, P. Although they are carnivorous and feed on many soft bodied marine animals, P.
Physalia has male and female colonies, which release their gametes into the water. The zygote develops into a single individual, which then buds asexually to form a new colony. Siphonophores include the largest known floating cnidarian colonies such as Praya dubia , whose chain of zoids can get up to 50 meters feet long. Other hydrozoan species are solitary polyps Hydra or solitary hydromedusae Gonionemus. One defining characteristic shared by the hydrozoans is that their gonads are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.
Section Summary Cnidarians represent a more complex level of organization than Porifera. They possess outer and inner tissue layers that sandwich a noncellular mesoglea between them. Cnidarians possess a well-formed digestive system and carry out extracellular digestion in a digestive cavity that extends through much of the animal. 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 absorb nutrients.
The gastrovascular cavity has only one opening that serves as both a mouth 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 environment, and between cells in the gastrodermis with water in the gastrovascular cavity. The lack of a circulatory system to move dissolved gases limits the thickness of the body wall, necessitating a non-living mesoglea between the layers.
There is no excretory system or organs; nitrogenous wastes simply diffuse from the cells into the water 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 about 10, 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.
Members of the class Anthozoa display only polyp morphology and have cnidocyte-covered tentacles around their mouth opening.
The class Anthozoa includes all cnidarians that exhibit a polyp body plan 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 6, described species.
Sea anemones are usually brightly colored and can attain a size of 1. These animals are usually cylindrical in shape and are attached to a substrate. Anthozoans : The sea anemone a , like all anthozoans, has only a polyp body plan b. The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. They have slit-like mouth openings and a pharynx, which is the muscular part of the digestive system that serves to ingest as well as egest food.
It may extend for up to two-thirds the length of the body before opening into the gastrovascular cavity. This cavity is divided into several chambers by longitudinal septa called mesenteries.
Each mesentery consists of one ectodermal and one endodermal cell layer with the mesoglea sandwiched in between. Mesenteries do not divide the gastrovascular cavity completely; the smaller cavities coalesce at the pharyngeal opening. The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange.
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey using the cnidocytes. 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 contained within the nematocysts.
Another type of anthozoan that forms an important mutualistic relationship is reef building 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 by producing gametes. Both gametes are produced by the polyp, which can fuse to give rise to a free-swimming planula larva.
The larva settles on a suitable substratum and develops into a sessile polyp. Scyphozoans are free-swimming, polymorphic, dioecious, and carnivorous cnidarians with a prominent medusa morphology. Class Scyphozoa, an exclusively marine class of animals with about 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 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. Scyphozoans : For jellyfish a , and all other scyphozoans, the medusa b is the most prominent of the two life stages. 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. The cell body of the choanocyte is embedded in mesohyl but protruding into the spongocoel is a mesh-like collar surrounding a single flagellum.
The beating of flagella from all choanocytes moves water through the sponge. Food particles are trapped in mucus produced by the sieve-like collar of the choanocytes and are ingested by phagocytosis.
This process is called intracellular digestion. Amoebocytes take up nutrients repackaged in food vacuoles of the choanocytes and deliver them to other cells within the sponge.
Despite their lack of complexity, sponges are clearly successful organisms, having persisted on Earth for more than half a billion years. Lacking a true digestive system, sponges depend on the intracellular digestive processes of their choanocytes for their energy intake.
The limit of this type of digestion is that food particles must be smaller than individual cells. Gas exchange, circulation, and excretion occur by diffusion between cells and the water. Sponges reproduce both sexually and asexually.
Asexual reproduction is either by fragmentation in which a piece of the sponge breaks off and develops into a new individual , or budding an outgrowth from the parent that eventually detaches.
A type of asexual reproduction found only in freshwater sponges occurs through the formation of gemmules , clusters of cells surrounded by a tough outer layer. Gemmules survive hostile environments and can attach to a substrate and grow into a new sponge. Sponges are monoecious or hermaphroditic , meaning one individual can produce both eggs and sperm.
Sponges may be sequentially hermaphroditic, producing eggs first and sperm later. Eggs arise from amoebocytes and are retained within the spongocoel, whereas sperm arise from choanocytes and are ejected through the osculum. Sperm carried by water currents fertilize the eggs of other sponges.
Early larval development occurs within the sponge, and free-swimming larvae are then released through the osculum. This is the only time that sponges exhibit mobility. Sponges are sessile as adults and spend their lives attached to a fixed substrate. Watch this video that demonstrates the feeding of sponges. The phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic. Nearly all about 99 percent cnidarians are marine species.
These cells are concentrated around the mouth and tentacles of the animal and can immobilize prey with toxins. Nematocysts contain coiled threads that may bear barbs.
The outer wall of the cell has a hairlike projection that is sensitive to touch. When touched, the cells fire the toxin-containing coiled threads that can penetrate and stun the predator or prey see [Figure 3]. Examples of the polyp form are freshwater species of the genus Hydra ; perhaps the best-known medusoid animals are the jellies jellyfish. Polyps are sessile as adults, with a single opening to the digestive system the mouth facing up with tentacles surrounding it.
Medusae are motile, with the mouth and tentacles hanging from the bell-shaped body. In other cnidarians, both a polyp and medusa form exist, and the life cycle alternates between these forms. All cnidarians have two tissue layers. The outer layer is called the epidermis , whereas the inner layer is called the gastrodermis and lines the digestive cavity. Between these two layers is a non-living, jelly-like mesoglea. There are differentiated cell types in each tissue layer, such as nerve cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as intercellular connections between the cells.
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