Organisms known as Scyphozoans (true jellyfish) make up the Class Scyphozoa. These organisms spend most of their life as medusae; they are larger than hydrozoans and are better swimmers. Additionally, jellyfish have photoreceptors that help them determine if it is dark or light. Most species avoid bright sunlight and will only surface around twilight or on cloudy days. Their tentacles can reach anywhere from 10 centimeters to 7-10 meters in diameter. Below are a few examples of different species of Scyphozoans or "true jellyfish."