Phylum Cnidaria• General Characteristics:

– Cnidarian means “stinging creature.”– Radial symmetry – Two different body plans exist:

• medusa and polyp – Cnidarians are made up of two tissue layers separated by mesoglea.

• Habitat: aquatic

Phylum Cnidaria• Nutrition: Capture pray using cnidocytes (stinging cells).– Sting prey and paralyze it so that it can be pulled into the

mouth to be digested.– Some organisms become immune to the sting and form

mutualistic relationships with cnidarians.

Phylum Cnidaria• Digestion: takes place in the gastrovascular cavity, a digestive

chamber with one opening, food enters here.– Break down occurs in the cavity and then partially

digested food is absorbed by the gastroderm and digestion is finished intracelluarly.

Phylum Cnidaria• Circulation: takes place through diffusion in the

gastrovascular cavity• Respiration: Diffusion of oxygen and carbon dioxide takes

place in the gastrovascular cavity.• Excretion: diffusion through the body wall or exits out

through the mouth of the organism.

Phylum Cnidaria

• Nervous: – Touch sensitive stinging cells

called nematocysts– a nerve net, but no brain– statocysts used to detect

gravity– ocelli to detect light

(eyespots)

Phylum Cnidaria

• Reproduction:– Sexual reproduction occurs through external fertilization

when the sperm is united with an egg outside of the body

Phylum Cnidaria

• Reproduction: – Asexual reproduction can occur through budding and

fission.

Phylum Cnidaria• Examples: sea anemones and coral have a dominant polyp

stage, hydra alternate between forms, jellyfish and sea wasps (box jelly) with a dominant medusa form