Phylum Cnidaria

Phylum Cnidaria

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.






Jelly Fish by Jman’s Skittles is licensed under CC BY 2.0

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