• Phylum Cnidaria

• Hydras, jellies, sea anemones, and corals

• Soft-bodied

• Carnivorous

• Stinging tentacles arranged in circles around their mouths

• Simplest animals to have body symmetry and specialized cells

• Stinging cells that are located on their tentacles

• Used for defense and to capture prey

• A poison-filled, stinging structure that contains a tightly coiled dart

• Found within cnidocytes

• Only a few cells thick

• Simple body systems

• Most of their responses to the environment are carried out by specialized cells and tissues

• Central mouth surrounded by numerous tentacles that extend outward from the body

• Life cycles includes a polyp and a medusa stage

• Polyp: cylindrical body with arm-like tentacles; mouth points upward

• Medusa: motile, bell-shaped body; mouth on the bottom

• Polyps and medusas have a body wall that surrounds an internal space: the gastrovascular cavity

• Gastrovascular cavity: a digestive chamber with one opening

– Food enters and wastes leave the body

• Following digestion, nutrients are usually transported throughout the body by diffusion

• Respire and eliminate wastes by diffusion through body walls

• Statocysts: groups of sensory cells that help determine the direction of gravity

• Ocelli: eyespots made of cells that detect light

• Specialized sensory cells are used to gather information from the environment

• Nerve net: loosely organized network of nerve cells that together allow cnidarians to detect stimuli – Distributed uniformly throughout the body in

most species – In some species it is concentrated around the

mouth or in rings around the body

• Hydrostatic skeleton: a layer of circular muscles and a layer of longitudinal muscles that enable cnidarians to move

• Polyps can reproduce asexually by budding

• External sexual reproduction – The sexes are separate-each individual is

either male or female

– Both egg and sperm are released into the

water

• Jellies (formerly jellyfishes)

• Hydras and their relatives

• Sea anemones

• Corals

• The worldwide distribution is determined by:

– Temperature

– Water depth

– Light intensity

• Many suffer from human activity

• Coral bleaching has become common

• Global warming may add to the problem

Cnidarians True tissues, but

only two germ layers (endoderm & ectoderm)

Radial symmetry Stinging cells called

nematocysts Gastrovascular cavity

(single opening) Examples:

Jellyfish Corals Anemones

Nematocyst

Thread (coiled)

CNIDOCYTE

“Trigger”

Discharge of thread

Cuticle of prey

Tentacle

Thread

Ectoderm

Endoderm

Mesoderm

Cnidarian 2 germ layers

3 germ layers