cnidaires

Cnidarians are a phylum of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. They are characterized by their radial symmetry and the presence of specialized stinging cells called cnidocytes, which they use to capture prey and defend themselves. These cells contain nematocysts, which are harpoon-like structures that can inject venom.

Cnidarians exhibit two primary body forms: the polyp, which is sessile and typically cylindrical, and the medusa, which is free-swimming and bell-shaped. Many species alternate between these two forms during their life cycle. For example, a polyp stage might bud off medusae, which then reproduce sexually, and the resulting larvae may settle to form new polyps.

The body of a cnidarian is very simple, consisting of a sac-like structure with a single opening that serves as both mouth and anus. This opening leads to a gastrovascular cavity where digestion takes place. They lack complex organ systems such as a brain, heart, or lungs. Their nervous system is a diffuse network of nerve cells that allows them to respond to stimuli.

Cnidarians inhabit a wide range of marine and freshwater environments, from shallow coastal waters to the deep sea. Corals, in particular, are known for their ability to build reefs, which are important ecosystems supporting a vast diversity of marine life. Their role in food webs is significant, as they prey on smaller organisms and are preyed upon by larger animals.