12 Questions
What is a characteristic of cnidarians' body structure?
Radial symmetry and two tissue layers
What is the main difference between polyps and medusae?
Polyps are stationary, while medusae move freely
What is the location of the mouth in a polyp?
On the top of the body
What is the shape of a medusa's body?
Umbrella-shaped with a fringe of tentacles
What is the main function of the gastrovascular cavity?
To circulate nutrients and oxygen to internal cells
What is a characteristic of the body cavity of cnidarians?
It is filled with a jelly-like substance
What is the purpose of the cnidocytes in cnidarians?
To capture prey and defend the animal
Which of the following cnidarians exist only as polyps?
Hydras and sea anemones
What is the purpose of muscle cell contractions in some polyps?
To move the body
What is unique about the cnidocytes of cubozoans?
They are more toxic than those of other cnidarians
What is a unique feature of some jellies?
Their long tentacles
What is the role of symbiotic algae in reef-building corals?
To provide energy through photosynthesis
Study Notes
Characterization of Cnidarians
- Cnidarians are characterized by radial symmetry and bodies arising from only two tissue layers.
- Their simple body has an outer epidermis and an inner cell layer that lines the digestive cavity.
- A jelly-filled middle region may contain scattered amoeboid cells.
- Contractile tissues and nerves occur in their simplest forms in cnidarians.
Body Forms of Cnidarians
- There are two kinds of radially symmetric body forms: polyp and medusa.
- Polyps have a cylindrical body with tentacles projecting from one end, exemplified by hydras and sea anemones.
- Medusae are shaped like an umbrella with a fringe of tentacles around the lower edge, exemplified by marine jellies.
Life Cycle of Cnidarians
- Some cnidarians pass sequentially through both a polyp stage and a medusa stage in their life cycle.
- Others exist only as medusae; still others, including hydras and sea anemones, exist only as polyps.
Feeding and Digestion
- Cnidarians are carnivores that use their tentacles to capture small animals and protists and to push the prey into their mouths.
- The mouth leads into a multifunctional compartment called a gastrovascular cavity, where food is digested.
- Fluid in the gastrovascular cavity circulates nutrients and oxygen to internal cells and removes metabolic wastes.
Unique Features of Cnidarians
- Phylum Cnidaria is named for its unique stinging cells, called cnidocytes, that function in defense and in capturing prey.
- Each cnidocyte contains a fine thread coiled within a capsule that can sting or entangle prey when discharged.
- Some large marine cnidarians use their stinging threads to catch fish.
Coral Animals
- Coral animals are polyp-form cnidarians that secrete a hard external skeleton.
- Each generation builds on top of the skeletons of previous generations, constructing the characteristic shapes of coral reefs.
- Reef-building corals depend on sugars produced by symbiotic algae to supply them with enough energy to maintain the reef structure.
Learn about the characteristics of phylum Cnidaria, including radial symmetry, body structure, and tissue layers. Discover the unique features of hydras, sea anemones, corals, and jellies.
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