Cnidae are stinging cells for defense and capture of prey.
Cnidae can be discharged only once.
Planula larvae are formed as a result of external fertilization.
Sequential polymorphism shows two life forms: polyp & medusa
An example which shows sequential polymorphism is Obelia
The polyp form of Obelia has both feeding and reproductive polyps.
Cnidaria are composed of 2 cell layers - an internal gastroderm and an external ectoderm.
Cnidaria can undergo asexual and sexual reproduction
Hydra, known only in the polyp form can undergo both asexual and sexual reproduction.
Sexual reproduction involves the formation of gonads in different individuals.
Classes within Cnidaria
Scyphozoa are known collectively as jellyfish.
Cubozoa have a dominant medusa stage.
Anthozoans include the sea anemones and corals, and are exclusively in the polyp form.
The skeletons of corals, composed of calcium carbonate, form coral reefs.
Hydrozoans may consist of numerous polyp forms living together as a colony.
Home Page: http://www.cbu.edu/~seisen/
Other information: Review of Cnidaria