Cnidaria and Nematocysts
Cnidaria, also called Coelenterata, is a large phylum of marine organisms with around 10,000 species. These species are corals, jellyfish, various plant-like organisms, as well as many others. These are all very different from each other, except for one similar attribute. They all have Nematocysts, which are stinging cells used by all Cnidaria for either defense or hunting. Nematocysts have a variety of functions to fulfill these tasks. The organism can use them by wrapping them around food or making it sticky so that the food is not able to escape. They can also use them in a more spear-like way of penetrating surfaces or secreting their poison into the water. Organisms such as jellyfish and sea anemones are widely known for their Nematocysts, which they use to capture fish for food. Nematocysts are very small, they cannot be seen by the human eye, and can only be activated once; afterward a new one grows and replaces the old.
Cnidaria are divided into four groups: Anthozoa, Cubozoa, Hydrozoa, and Scyphozoa. Anthozoa are the corals that make up coral reefs as well as sea anemones and various other similar organisms. These date back to the late Precambrian era, but the first of these to be coral-like arrived in the Cambrian era and it was only until the middle of the Triassic that Corals we are familiar with came to being.
Cubozoa, or box jellies, look very similar to normal jellyfish (apart from being square from top-down), but are very different. What separates these from jellyfish is that while not having a brain, they can swim fast and maneuver around and towards obstacles, something jellyfish cannot do. Another characteristic that separates these creatures from jellyfish is that some species have eyes to help navigate. Because these have no hard parts, there is no fossil record to give a date of when they originated.
Hydrozoa are animals in which during their life cycle they change between polyp and medusa...