Phylum Cnidaria

Includes jellyfish, anemones, and other marine animals.
One of the most ancient animal phyla, with fossils dating back over 700 million years.
Prefer shallow, warm waters.
Almost exclusively marine, with some freshwater species.

Symmetry: Radial or biradial.
Tissues: Diploblastic (two tissue layers).
Gastrovascular cavity (GVC): Serves as a stomach where food enters and waste exits through the same opening.
Defining Characteristic: Cnidocyte (stinging cell), located on tentacles and around the mouth.

Cnidocytes contain nematocysts, which are organelles that contain:
- A barbed thread for capturing prey.
- A cnidocil, a trigger that deploys the barb when activated by touch or chemicals.
- The thread contains toxins that can immobilize prey, which may include neurotoxins or toxins that lyse cells.
Nematocysts can only be used once and need to be regenerated after firing.
Tentacles carry dead prey into the GVC for digestion.

Body Types: Two main forms - Polyp and Medusa.
- Polyp: Sessile as adults but motile as larvae.
- Medusa: Free-swimming, motile.
Some species exhibit dimorphism, possessing both polyp and medusa forms.
Tissue Structure:
- Epidermis derived from ectoderm.
- Gastrodermis lines the GVC and is derived from endoderm.
- Non-living mesoglea tissue lies between these layers.
Possesses specialized, differentiated tissue but lacks true organ systems.

Nervous System: Rudimentary, consisting of a nerve net that is more advanced in medusa than in polyp species.
Nerve cells possess both motor and sensory capabilities, with peptides as signaling molecules.
Respiration: No specialized respiratory system; O2 is absorbed and CO2 released through the gastrovascular cavity and epidermis.
Circulatory System: Lacks a circulatory system; body is typically only two cell layers thick with mesoglea in between.
Excretory System: Absence of a dedicated excretory system; nitrogenous waste diffuses into the GVC or surrounding water.

Capable of asexual reproduction, typically through budding.
Capable of sexual reproduction; some species are monoecious (both sexes in one) or dioecious (separate sexes).
Medusa release eggs and/or sperm into water for external fertilization.
Zygote develops into a ciliated, bilaterally symmetrical planula larva, which eventually forms a polyp.

Class Anthozoa: Only sessile polyp species, includes corals, sea fans, and sea anemones.
Class Scyphozoa: Represents medusa species; commonly known as jellyfish.
Class Cubozoa: Another medusa class, includes box jellies.
Class Hydrozoa: Includes both sessile and medusa species like hydra and the Portuguese man o’ war.