Cnidarians (Phylum Cnidaria) [nahy-dair-ee-uh] LEQ: What are the two body forms of Cnidarians?
Cnidarians • Sometimes called coelenterata [si-len-tuh-rey-tuh]. Include the sea anemones, jellyfishes, and corals. More than 9,000 species are known, almost all of which are marine.
Cnidarians • First animals to have tissues. Tissues allowed for locomotion, responding to stimuli, and engulfing prey. Body Plan: • Display radial symmetry (look the same from all sides and have neither a head nor a front or back).
Cnidarians • Can occur as one of two basic forms: a polyp (saclike attached stage) or a medusa (which is like an upside-down bowl adapted for swimming). Polyp stage Medusa Stage
Cnidarians • The life history of some cnidarians include both polyp and medusa stages (jellyfish, for example). Others spend their entire lives as one of the two.
Cnidarians • The polyp and medusa share a similar body plan. • Both have a centrally located mouth surrounded by tentacles, which are used to capture and handle food.
Cnidarians Feeding and Digestion • The mouth opens into a gut where food enters and digestion begins. The cnidarian’s gut does not end with an anus. Their gut is usually termed the gastrovascular cavity.
Cnidarians • Practically all cnidarians are predators, and must capture their prey. Their tentacles are lined with cells containing small, fluid-filled capsules containing a thread able to be quickly ejected. The thread can be sticky or armed with spines or consist of a long tube that wraps around parts of the prey.
Cnidarians • The capsules (cells) are called cnidocytes [nahy-duh-sist]. These cells contain organelles called nematocyst [nem-uh-tuh-sist]. Some nematocysts contain venom.
Box Jellyfish • So poisonous it can cause hyperkalemia which can lead to cardiovascular collapse and death as quickly as 2-5 minutes.
Cnidarians • Two layers of cells form the body wall of cnidarians: the endoderm and ectoderm. A jellylike material can be found in between the two cell layers. This substance is composed largely of water and is called mesoglea. In the medusa forms, the mesoglea makes up most of the body wall.
Cnidarians • The ectoderm cells contain contractile fibers. Locomotion is accomplished by contraction of these fibers. However, for the medusa (swimming) forms, the strength of these contractions is not great enough to overcome water currents. Thus, medusae drift.
Cnidarians • The endoderm cells line the gastrovascular cavity. These cells release digestive enzymes into the cavity to begin the digestion process. • When the food is partially digested, it is engulfed by the endoderm cells, where digestion is completed within food vacuoles. • Thus, digestion is both extracellular and intracellular.
Cnidarians Nervous and Respiratory Systems • Though cnidarians lack a brain or true nerves, they do have specialized nerve cells. These cells interconnect to form a nerve net that transmits impulses in all directions. • This simple nervous system can produce some relatively sophisticated behaviors. For instance, some corals can recognize and kill “imposters” in the colony.
Cnidarians • Cnidarians have no respiratory system or circulatory system. They obtain their oxygen and excrete their wastes by diffusion.
Cnidarians • Classes of cnidarians include: • The hydrozoans (class Hydrozoa): hydroids (feathery or bushy colonies of tiny polyps) and siphonophores (hydrozoans that form drifting colonies, such as the Portuguese man-of-war) • The scyphozoans (class Scyphozoa): jellyfishes • The anthozoans (class Anthozoa): sea anemones and hard and soft corals.
What are the two body forms of Cnidarians? Polyp Medusa
1 extra credit point if you can tell me its name • Tentacool (jellyfish)