Earth Science: Invertebrates – Cnidarians
Common examples of the phylum Cnidaria include corals and jellyfish. Organisms live either individually or in colonies. Their tubular or bell- shaped bodies are armed with stinging cells.
The bodies of cnidarians primarily consist of two cell layers lining the mesoglea, which is a noncellular layer in between. These layers enclose the gastrovascular cavity, where digestion occurs. An opening links this hollow space to the outside and allows food particles to enter and waste products to exit.
Cnidarians may occur as more or less stationary polyps or as medusae, or “jellyfish,” which can swim freely. Both types of cnidarians undergo cyclic changes in appearance from one generation to the next. For example, true jellyfish, such as the familiar bell-shaped jellyfish, produce eggs that develop into larvae.
After a period of swarming, they will attach to a suitable surface and grow into a polyp. After reaching a certain size, the polyp will pinch off radial discs that then develop into medusae. Not all cnidarians reproduce sexually; some reproduce asexually, usually by budding. There are about 9,000 species, varying in size from a small fraction of an inch (a few millimeters) up to about six feet (two m).
Coral
Coral belong to the taxonomic class of the Anthozoa. The life cycle of these barrel- shaped polyps does not include a generation as medusae—instead, the larvae grow into new polyps straightaway.
Coral usually have a basal plate that firmly attaches to the ground. Tentacles with toxic nemato- cysts surround the mouth to catch prey items and attack other predators.
Jellyfish
Almost all larger species of jellyfish belong to the taxonomic class Scyphozoa. Among them is the giant jellyfish Nemopilema nomurai, which measures up to about six feet (two m), not including the long tentacles. This giant medusa weighs as much as 440 pounds (200 kg).
The jellyfish bell consists of up to 99 percent water. Elastic tentacles with stinging cells hang from the edge of the bell. Inside the transparent body, the often colorful gonads are quite visible. In contrast, the corresponding polyps are relatively small.
MINIATURE HARPOONS
Cnidarians have specialized cells for predation or protection. The most simple version of these stinging cells consists of a double-walled bladder filled with toxic and stinging substances and covered by a lid.
The cavity also contains a threadlike stinging trigger coiled up in a spiral form and equipped with stiletto or spike bristles at the tip. Touching the tip re- leases the venom through the bristles, projecting it into the prey at high speed.
ISSUES TO SLOVE
CORAL REEFS are threatened with extinction. Greenhouse gas emissions cause carbon dioxide to dis-solve in the ocean, increasing its acidity. Coral cannot survive in acidic water.