Phylum Cnidaria

General Characteristics

• They are radially symmetrical

• They have 2 tissue layers:• Epidermis - Outer layer of

cells

• Gastrodermis Inner layer which lines the gut cavity or gastrovascular cavity

• In between these tissue layers is a non-cellular jelly-like material called mesoglea

Cnidarian Body Plans

Polyp form• Tubular body, with the mouth directed upward.• Around the mouth are a whorl of feeding tentacles.• Only have a small amount of mesoglea• Sessile Medusa form• Bell-shaped or umbrella shaped body, with the mouth is

directed downward.

• Small tentacles, directed downward.

• Possess a large amount of mesoglea

• Motile, move by weak contractions of body

Hydras, sea anemones, corals

Skeletal/Muscle System

• most have primitive muscle cells; coral have external skeleton of calcium carbonate

Stinging Organelles

• cnidarians are carnivores (eat meat) that use tentacles arranged in a ring around their mouth to capture prey

• Prey is captured by using specialized stinging cells called cnidocytes located in the outer epidermis.

• Each cnidocyte has a modified cilium - cnidocil, and is armed with a stinging structure called a nematocyst.

• Some nematocysts function to entangle the prey; others harpoon prey and inject a paralyzing toxin.

Digestion• Sac w/ a digestive cavity (gastrovascular

cavity) w/ a single opening that acts as a mouth/anus

Jellyfish Hydra

Digestion (cont.)

• food is pushed into the gastrovascular cavity where digestion begins (extracellular digestion)

• undigested remains leave through the mouth/anus

Nervous System

• Nerve Net- Cnidarians do not have a brain but they have simple sensory receptors that detect & respond to stimuli

Circulation

• NONE!!

Respiration

• Diffusion- oxygen from water moves into sponge cells (high to low)

Reproduction

• Asexual – budding- formation of outgrowths that pinch off from parent to live independently

• Sexual- forms a zygote and free swimming planula that settles on ocean floor

• *MOST ALTERNATE B/W POLP & MESDUSA LIFE FORM

Reproduction (cont.)

Reproduction (cont.)

Excretion

• mouth/anus- release waste