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Phylum Cnideria. Next level of organizational complexity – these animals can swim, respond to external stimuli and engulf prey. Phylum: Cnidaria – sometimes called Coelenterata. This phylum includes sea anemones, jellyfishes, corals and their relatives.
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Phylum Cnideria Next level of organizational complexity – these animals can swim, respond to external stimuli and engulf prey.
Phylum: Cnidaria– sometimes called Coelenterata • This phylum includes sea anemones, jellyfishes, corals and their relatives. • Cnidarians display radial symmetry, where similar parts of the body are arranged and repeated around a central axis. Animals with radial symmetry look the same from all sides and have no head, front, or back. • They do have an oral surface – the area where the mouth is. They also have an aboral surface on the opposite side.
Cnidarians occur in one of two basic forms: • A polyp – a sac-like phase which is attached to a substrate. • A medusa – a bell-like or jellyfish shape which is like an upside-down polyp adapted for swimming. • The polyp and the medusa share a similar body plan. • A centrally located mouth surrounded by tentacles • The mouth that opens into a gut where food is digested. The gut has only one opening, the mouth, so what is not digested has to go out that way. • All Cnidarians have nematocysts – unique stinging structures found on the tentacles that capture food.
Almost all Cnidarians are carnivores-animals that prey on other animals. • Nematocysts are used primarily to capture prey. They have a fluid filled capsule containing a thread that can be quickly ejected. The thread may be sticky or armed with spines, or be a long tube that wraps around parts of the prey. • Some nematocysts contain toxins. • Cnidarians do not have a brain or true nerves. They do have a nerve net of cells that transmits impulses in all directions.
Class: Hydrozoa • Hydrozoans have a variety of forms and life histories. • Many have feathery or bushy colonies of tiny polyps. They attach to pilings, shells, seaweeds, and other surfaces. • The polyps may be specialized for feeding, defense, or reproduction. • The reproductive polyps produce minute, transparent medusae. The medusae, usually planktonic, release gametes. The fertilized eggs develop into free-swimming larvae call a planula.
Siphonophores are hydrozoans that form drifting colonies of polyps. Some polyps in the colony may be specialized as floats, which may be gas-filled, as in the Portuguese man-of-war (Physalia physalis). • Other siphonophore polyps form long tentacles used to capture prey. • Toxins from the nematocysts can produce painful reactions to swimmers and divers.
Class Scyphozoa • Scyphozoans are larger jellyfishes common in all oceans. • The medusae are the dominant stage of the life cycle. • The polyp stage of scyphozoa are very small and release juvenile medusae. • The bell of some scyphozoa may reach a diameter of 6 ½ feet in some species. • Scyphozoans can swim using rhythmic contractions of the bell, but are easily carried around by currents. • Some scyphozoans are among the most dangerous marine animals known, giving extremely painful and sometime fatal stings.
Class: Anthozoa • Anthozoans are solitary or colonial polyps that do not have a medusa stage. • Animals of the class anthozoa are sea anemones, corals, soft corals, sea pens, and sea fans. • Sea anemones have large polyps and corals have very small polyps with calcium carbonate skeletons that form large reef structures.
Cniderian Terminology to Know • Nematocyst • Radial symmetry • Oral surface • Aboral surface • Polyp • Medusae • Tentacles • Toxins • Gastrovascular cavity
