Which hydrozoan predominates in the medusa stage

The Anthozoa contain only sessile polyp forms, while the Medusozoa include species with both polyp and medusa forms in their life cycle. Sea anemones are usually brightly colored and can attain a size of 1.

Individual animals are cylindrical in shape and are attached directly to a substrate. The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. The slit-like mouth opening and flattened pharynx are lined with ectoderm. This structure of the pharynx makes anemones bilaterally symmetrical. A ciliated groove called a siphonoglyph is found on two opposite sides of the pharynx and directs water into it.

The pharynx is the muscular part of the digestive system that serves to ingest as well as egest food, and 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 a fold of gastrodermal tissue with a layer of mesoglea between the sheets of gastrodermis.

Mesenteries do not divide the gastrovascular cavity completely, and 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, as well as additional mechanical support for the body of the anemone. Sea anemones feed on small fish and shrimp, usually by immobilizing their prey with nematocysts.

Some sea anemones establish a mutualistic relationship with hermit crabs when the crab seizes and attaches them to their shell. In this relationship, the anemone gets food particles from prey caught by the crab, and the crab is protected from the predators by the stinging cells of the anemone. Some species of anemone fish, or clownfish, are also able to live with sea anemones because they build up an acquired immunity to the toxins contained within the nematocysts and also secrete a protective mucus that prevents them from being stung.

The structure of coral polyps is similar to that of anemones, although the individual polyps are usually smaller and part of a colony, some of which are massive and the size of small buildings. Coral polyps feed on smaller planktonic organisms, including algae, bacteria, and invertebrate larvae. Some anthozoans have symbiotic associations with dinoflagellate algae called zooxanthellae.

The mutually beneficial relationship between zooxanthellae and modern corals—which provides the algae with shelter—gives coral reefs their colors and supplies both organisms with nutrients.

This complex mutualistic association began more than million years ago, according to a new study by an international team of scientists. That this symbiotic relationship arose during a time of massive worldwide coral-reef expansion suggests that the interconnection of algae and coral is crucial for the health of coral reefs, which provide habitat for roughly one-fourth of all marine life. Reefs are threatened by a trend in ocean warming that has caused corals to expel their zooxanthellae algae and turn white, a process called coral bleaching.

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. 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. They are small compartments that house a mini needle-like stinger. Do cnidarians have a nervous system? The nerve net is the simplest form of a nervous system found in multicellular organisms.

Hydra, which are cnidarians, have a nerve net throughout their body. On the other hand, sea stars, which are echinoderms, have a nerve net in each arm, connected by a central radial nerve ring at the center. Are cnidarians deadly to humans?

All cnidarians have the potential to affect human physiology owing to the toxicity of their nematocysts.

Most are not harmful to humans, but some can impart a painful sting—such as Physalia, the Portuguese man-of-war, and sea anemones of the genus Actinodendron. How do jellyfish reproduce? Jellyfish reproduction involves several different stages. In the adult, or medusa, stage of a jellyfish, they can reproduce sexually by releasing sperm and eggs into the water, forming a planula.

The polyps clone themselves and bud, or strobilate, into another stage of jellyfish life, called ephyra. What are the only fully formed organs on cnidarians? Cnidarians also have a sac body plan with a gastrovascular cavity, and the water within the gastrovascular cavity acts as a hydrostatic skeleton. There are no true organs, and adult structures form in the diploblastic ectoderm and endoderm tissues. What is a hold down anchor? What are the hot peppers at In N Out?

Planulae are radially symmetric ovoids, often covered with flagellate cells for swimming. They may be very simple embryos or have cells differentiated into several types. Planulae most often settle onto a benthic substrate and develop into a polyp. Polyps are radially symmetric, and may be urn-shaped, conical, cylindrical, or club-shaped. In most species they are only a few millimeters tall, though the largest grow up to many centimeters, and one, Branchiocerianthus imperator can be 2 meters tall.

At their base hydrozoan polyps have basal disks or elongate processes for attaching to substrate, or they may be attached to other polyps. Often there will also be connections here to hollow tubes called stolons that connect the polyp to others in its colony, and allow the exchange of food between polyps. Above the base is a ring of contractile cells called the sphincter. These can contract to isolate the contents of the polyp from the stolons, preventing undigested food from entering the stolons.

Above this is the gastric column, which usually contains a digestive chamber with a single opening, a mouth at the apex of the column. A ring of tentacles is attached to the column below the apex and above the sphincter. The number, shape and size of tentacles varies greatly, but there are usually between and 8 and 50 on a single polyp some have many more, and some specialized polyps may have fewer. Most colonial hydrozoans are polymorphic, with different structures reflecting different functions.

Some are armed with large spines tentacles for defense but have no mouth, some have tentacles and functional mouths for feeding, and some are only reproductive, with no tentacles or mouth, and produce medusae see below or gametes. Like all cnidarians , hydrozoans have special ectodermal cells called cnidocytes, each containing a single intracellular structure called a cnida aka nematocyst. Cnidae are unique to the Cnidaria.

Each cnida, when triggered by a mechanical or chemical stimulus, shoots out a tiny hollow tube at high speed. Hydrozoans use different types of cnidae to capture prey, to repel predators, and to attach to substrate.

Most hydrozoan species are colonial. A founding polyp produces new polyps by budding, and these grow a network of interconnecting hollow tubes stolons formed of living tissue, collectively called the coenosarc. Colony growth forms vary between species, some may form a single layer of polyps spreading across the substrate, others growing as erect stems, with polyps growing off the stems.

Polyps and the coenosarc may secrete chitinous sheaths, or stems, or calcareous coatings the latter forming structures similar to the anthozoan Scleractinia , the stony corals. In many colonies, polyps are polymorphic, with different structures reflecting different functions. Some have no mouth, but are armed with large spines or cnidae-equipped tentacles for defense, some have tentacles and functional mouths for feeding, and some, with neither mouth nor tentacles, are strictly reproductive, and produce medusae see below or gametes.

The medusa is the sexually reproducing stage in most hydrozoans. They are often formed by budding from polyps, and are usually solitary free-swimming organisms. They are similar in structure to an inverted polyp, radially symmetric, and often have four-fold symmetry.

Their main body part is the umbrella, a bell or cone shaped gelatin-filled structure, which floats with the opening down. Medusa are usually small, usually mm in diameter, though a few are larger, the largest genus g. Rhacostoma grow to mm in diameter. Around the inside of the opening is a muscular ring of tissue called the velum. The velum can contract and relax, changing the diameter of the opening, and playing an important role in swimming The presence of the velum is a diagnostic character for Hydrozoa, only one genus, Obelia , has lost it.

Around the outside of the opening of the umbrella is a ring of tentacles, which vary greatly among species in number, shape, and degree of arming with cnidocytes. Inside the umbrella, suspended like the clapper of a bell, is the manubrium, which contains the gastric cavity, and ends in a mouth.

Structures that produce gametes form on the sides of the manubrium. Most species have dioecious medusae, each individual producing only eggs or sperm. Some are monoecious, but usually not simultaneously hermaphrodite. In some species sex is determined by environmental conditions, mainly temperature. Both polyps and medusae have networks of nerves, but no brain or central ganglion. Some have light-sensitive structures called ocelli, and many have statocysts that allow them to detect gravity and their orientation.

These structural patterns are common, but there is great variation in the life cycles of hydrozoans. Some have suppressed or reduced one or more stages. In the Siphonophora and a few other groups of hydrozoans, colonies of polyps are pelagic, and float at the surface by means of a gas-filled tissue.

They often retain medusae as part of the colony. Hydrozoans have a complex life cycle, usually with two or three morphologically different stages. The classic cycle starts with fertilized eggs developing into small, free swimming larvae called planulae, which may be able to enter a dormant resting state to resist unsuitable environmental conditions.

Planulae transform into sessile polyps, usually attached to substrate, but free-floating in some groups. Polyps duplicate themselves asexually by budding, often producing colonies of hundreds or thousands of polymorphic individual polyps. Polyps produce "adult" sexually-reproducing medusae by budding. Medusae are solitary, free-swimming, dieocious. They release sperm and eggs into the water, where fertilization occurs. This is the basic cycle, but there is an enormous range of variations.

In nearly half of species e. Hydra the the medusa stage is entirely suppressed; polyps produce gametes directly.