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. Hydras, sea anemones, corals • Small tentacles, directed downward. • Possess a large amount of mesoglea • Motile, move by weak contractions of body
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 cnidocyteslocated 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
Excretion • mouth/anus- release waste