How does cnidaria obtain food

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.

However, organs and organ systems are not present in this phylum. The nervous system is primitive, with nerve cells scattered across the body in a network. The function of the nerve cells is to carry signals from sensory cells and to contractile cells. Groups of cells in the nerve net form nerve cords that may be essential for more rapid transmission.

Cnidarians perform extracellular digestion , with digestion completed by intracellular digestive processes. Food is taken into the gastrovascular cavity , enzymes are secreted into the cavity, and the cells lining the cavity absorb the nutrient products of the extracellular digestive process.

The gastrovascular cavity has only one opening that serves as both a mouth and an anus an incomplete digestive system. Like the sponges, Cnidarian cells exchange oxygen, carbon dioxide, and nitrogenous wastes by diffusion between cells in the epidermis and gastrodermis with water. The phylum Cnidaria contains about 10, described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa.

The class Anthozoa includes all cnidarians that exhibit a sessile 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. 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. Diffusion is the movement of a material red circles from regions of higher concentration Side A to regions of lower concentration Side B until equilibrium is reached same number of particles move from Side A to Side B as move from Side B to Side A.

Diffusion is a natural process caused by the random movement of particles but is only useful to very small or thin organisms ones which have a large surface area compared to their volume where the material in higher concentration does not have far to move.

Cnidaria do not have a brain or groups of nerve cells " ganglia ". They do not have a head, but they have a mouth, surrounded by a crown of tentacles. The tentacles are covered with stinging cells nematocysts. Nearly all cnidaria are predators, their nematocysts can paralyze and kill prey much larger than them.

Small sensory hairs near the nematocysts are sensitive to vibrations in the water. Any prey swimming past can trigger the nematocyst which shoots out the barb. This penetrates the prey's outer covering and injects it with venom. The prey is then moved to the mouth by a tentacle. Their prey can range in size from plankton to animals several times larger than themselves.

Some obtain their energy from algae that that live in their bodies and a few are parasites. Other Cnidaria, including the corals, get their nutrients from symbiotic algae within their cells.

Predators of Cnidaria include sea slugs, sea stars for example, the Crown of Thorn which can devastate coral reefs , nudibranchs, fish including butterfly fish and parrot fish, which eat corals and marine turtles and sunfish, which eat jellyfish.

Plastic bags floating in the oceans are dangerous to turtles which often can mistake them for jellyfish and their guts can become blocked by them. For example coral, inside the sac of each coral polyp lives a singlee-celled algae called zooxanthellae. The algae produces oxygen and energy sugars that the coral polyp needs to live and, in return, the polyp produces carbon dioxide and other substances the algae needs. Sea anemones and coral have a body shape known as a polyp, and are generally stationary, while jellyfish and hydra are medusae, and are able to move through the water.

With these differences, it might come as no surprise that each species of cnidarian has different eating habits. Sea anemones are polyp-shaped, sedentary sea creatures that tend to resemble flowers. However, the "petals" on these underwater flowers are actually tentacles that contain stingers, and the "stalk" is basically one large stomach. Although they're unable to go hunting, sea anemones are no less predatory creatures that eat any plankton or small fish that are unlucky enough to swim too close to their tentacles and get stung.

The tentacles push the small fish into the anemone's mouth, and after the unfortunate animal is digested, the waste exits the same way. Another type of polyp cnidarian is coral, which tend to be smaller and live together in colonies. Coral reefs are actually the skeletons of these cnidarians that have built up over time.