Subphylum Medusozoa
– Class Scyphozoa
– Class Cubozoa
– Class Hydrozoa
Subphylum Anthozoa (used to be Class)
– Subclass Hexacorallia
– Subclass Octocorallia

+ Contains over 10,000 species of animals found exclusively in aquatic (freshwater and marine) environments.

+ Includes four groups: Scyphozoans, Hydrozoans, Cubozoan, and Anthozoans

Anthozoa: True corals, anemones, and sea pens
Cubozoa: Box jellies with complex eyes and potent toxins (Highly Toxic Nematocysts)
Hydrozoa: The most diverse group of siphonophores, hydroids, fire corals, and many medusae
Scyphozoa: The true jellyfish. (Most are Dioecious)

+ General Cnidarian characteristics

• Radially Symmetric
• Diploblastic (epidermis, gastrodermis)
• No cephalization, No central nervous system
• No special respiratory, excretory or circulatory organs
• Primarily Carnivores
• Polyp and Medusa body forms

+ Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Cnidocytes have
organelles called nematocysts. There are 6 different types of nematocysts. 

+ All Cnidarians have tentacles with stinging cells in their tips which are used to capture and subdue prey

+ They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration.

+ Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both.

+ Cnidarians’ activities are coordinated by a decentralized nerve net and simple receptors.

+ Not all cnidarians reproduce sexually, many of them have complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage.

+Have no separate gut, (just GV cavity/body wall) and only one body opening. This has several consequences:

• Food and undigested waste pass through the same opening in different directions
• Movements of body cause distortion of GV cavity
• GV cavity serves for both circulation and digestion
• Gametes and embryos must be released through the same opening

+ They have Hydrostatic Skeleton which;

• Uses fluid (water) in a body compartment for support and transmission of muscular forces
• Fluid is incompressible, force generated by displacement of fluid in one region can be used to do work in another

+ Many Cnidarians have endosymbiotic dinoflagellates (zooxanthellae) within their cells which is a very important mutualism (Ex, Sea anemone). While Coral polyps have endosymbiotic zooxanthellae (algae).

+ Anthozoan polyps are larger and more complex than Hydrozoan polyps with no Medusa stage. Its subclass Octocorallia have 8 pinnate tentacles and some are called Gorgonians (sea fans & sea whips). The other subclass Hexacorallia, have many tentacles, in multiples of 6. 

+ Class Scyphozoa gastric filaments on the gonads secrete enzymes and then phagocytize partially digested food. Digestion occurs intracellularly within food vacuoles.