What does the cniderian look like?

The cniderian body plan is characterized by radial symmetry, a gastrovascular cavity, and specialized stinging cells called nematocysts. They show a diploblastic level of organization, meaning they have two primary tissue layers: the ectoderm and the endoderm, with a mesoglea in between. Cnidarians exhibit two main body forms: polyps and medusae, which may alternate in their life cycle known as alternation of generations.

- Polyps: Polyps are sessile, meaning they are attached to a substrate. They have a cylindrical body with a mouth and tentacles at the top. The mouth leads to the gastrovascular cavity, which serves for both digestion and circulation. Polyps often form colonies through asexual reproduction. Examples of cnidarians that exist as polyps include corals, sea anemones, and hydroids.

- Medusae: Medusae are free-swimming and have a bell-shaped or umbrella-shaped body. They possess a mouth on the underside of the bell, surrounded by tentacles. Medusae propel themselves by rhythmic contractions of their bell-shaped body. They often have a complex life cycle involving both asexual and sexual reproduction. Jellyfish and some colonial species like Portuguese man o' war exhibit the medusa body form.

Overall, the appearance of a cniderian can vary widely depending on whether it exists as a polyp or medusa, as well as the specific species. They display diverse colors, shapes, and sizes, ranging from tiny hydroids to large jellyfish with long, trailing tentacles.