Cnidarians

Phyllum Cnidaria

Cnidaria: means “nettle.”Includes Jellyfish, Sea Anemones, and CoralsContains 9,000 mostly marine speciesCarnivorous: eat crustaceans, fishes, worms,

and diatoms.

3 Classes

Hydroidea: Small, polymorphic

(can be both medusa and polyps)

Scyphozoa: Mainly jellyfish Polymorphic

Anthozoa: Corals and sea

anemones. Lack medusa stage

Cnidoblasts

Almost all Cnidarians have stinging cells called cnidoblasts or nematocysts.

These cnidoblasts are deployed on tentacles that end or retract toward the mouth.

Each cnidoblast has a capsule with a coiled threat that can be forcefully ejected and contains a toxin or immobilizing prey.

Digestion

The tentacles bring food toward the saclike digestive cavity.

Digested food is absorbed by cells of the digestive cavity and transported to other parts of the animal by migratory cells or by diffusion.

Indigestible bones are eliminated through the mouth.

Morphology

MulticelluarSingle body cavity

called and enteronAn opening

surrounded by tentacles with stinging cells or cnidoblasts.

The opening functions as both the mouth and the anus.

Morphology

Three layers of cells: Inner layer called Endoderm or gastrodermis responsible for digestion

and reproduction. Outer layer called the ectoderm or epidermis for capturing prey and

for protection from attack. Mesogloea: gelatinous, non-living layer holding the other layers

together. Radial Symmetry: Body parts radiate from a central axis. No cnidarian possesses a definite head or concentration of

sensory receptors, but can have a primitive network of nerves.

No excretory or circulatory systems.Sea anemones and corals are benthic and attach to rocks.

Jellyfish are pelagic and float in the water column.