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. A mouth opening is surrounded by tentacles bearing cnidocytes [Figure 5]. Scyphozoans include all the jellies and are motile and exclusively marine with about described species.
The medusa is the dominant stage in the life cycle, although there is also a polyp stage. Species range from 2 cm in length to the largest scyphozoan species, Cyanea capillata , at 2 m across. Jellies display a characteristic bell-like body shape [Figure 6]. Identify the life cycle stages of jellies using this video animation game from the New England Aquarium. Cubozoans are anatomically similar to the jellyfish.
A prominent difference between the two classes is the arrangement of tentacles. Cubozoans have 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. Cubozoans typically exist in a polyp form that develops from a larva.
The polyps may bud to form more polyps and then transform into the medusoid forms. Watch this video to learn more about the deadly toxins of the box jellyfish. Hydrozoa includes nearly 3, species, 1 most of which are marine. Most species in this class have both polyp and medusa forms in their life cycle. Many hydrozoans form colonies composed of branches of specialized polyps that share a gastrovascular cavity. Other species are solitary polyps or solitary medusae.
The characteristic shared by all of these species is that their gonads are derived from epidermal tissue, whereas in all other cnidarians, they are derived from gastrodermal tissue [Figure 7] ab. Section Summary Animals included in phylum Porifera are parazoans and do not possess true tissues. These organisms show a simple organization.
Sponges have multiple cell types that are geared toward executing various metabolic functions. Cnidarians have outer and inner tissue layers sandwiching a noncellular mesoglea. Cnidarians possess a well-formed digestive system and carry out extracellular digestion. 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. These forms may produce additional polyps by budding or may transform into the medusoid form.
Nematocysts can inject toxins into their prey. Pelagic: living in the waters of the open ocean. Skip to main content. Cnidarian Features Two tissue layers with nerve and muscle cells Nematocysts, which are structures contained in special stinging cells that can catch food or defend from predators Two main life forms: free-swimming medusa e.
Sinauer Associates, Sunderland, MA. To cite this page: Myers, P. Disclaimer: The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe.
Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control. Cnidaria corals, sea anemones, jellyfish, and relatives Facebook.
Sources : Hickman, C. Animal Diversity. Brown, Dubuque, IA.
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