26-3 Cnidarians Slide 2 of 47

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2 What Is a Cnidarian? What is a cnidarian? 3 of 47

3 What Is a Cnidarian? What Is a Cnidarian? Cnidarians are soft-bodied, carnivorous animals that have stinging tentacles arranged in circles around their mouths. They are the simplest animals to have body symmetry and specialized tissues. 4 of 47

4 What Is a Cnidarian? Cnidarians get their name from the cnidocytes, or stinging cells, located along their tentacles. Cnidarians use cnidocytes for defense and to capture prey. 5 of 47

5 What Is a Cnidarian? Within each cnidocyte is a nematocyst a poison-filled, stinging structure that contains a tightly coiled dart. Trigger Filament Nematocyte Barb Filament 6 of 47

6 What Is a Cnidarian? When a shrimp or small fish brushes up against the tentacles, thousands of nematocysts explode, releasing enough poison to paralyze or kill the prey. 7 of 47

7 What two body plans exist in the cnidarian life cycle? 8 of 47

8 Cnidarians are radially symmetrical. They have a central mouth surrounded by numerous tentacles that extend outward from the body. 9 of 47

9 Cnidarians typically have a life cycle that includes two different-looking stages: a polyp and a medusa. Polyp Medusa 10 of 47

10 A polyp is a cylindrical body with armlike tentacles. In a polyp, the mouth points upward. Polyps are usually sessile. Polyp 11 of 47

11 A medusa has a motile, bell-shaped body with the mouth on the bottom. Medusa 12 of 47

12 Cnidarian polyps and medusas each have a body wall that surrounds an internal space called a gastrovascular cavity. Gastrovascular cavity Gastrovascular cavity 13 of 47

13 The gastroderm is the inner lining of the gastrovascular cavity, where digestion takes place. Gastrovascular cavity Gastrovascular cavity 14 of 47

14 The epidermis is the outer layer of cells. Epidermis Epidermis 15 of 47

15 The mesoglea is a layer that lies between the epidermis and gastroderm. Mesoglea 16 of 47

16 Feeding A cnidarian pulls its food through its mouth and into its gastrovascular cavity, a digestive chamber with one opening. Food enters and wastes leave the body through that same opening. 17 of 47

17 The digestion that occurs in the gastrovascular cavity is extracellular, meaning that it takes place outside of cells. Partially digested food is absorbed by the gastroderm. 18 of 47

18 Digestion is completed intracellularly, within cells in the gastroderm. Any materials that cannot be digested are passed out of the body through the mouth. 19 of 47

19 Respiration, Circulation, and Excretion Following digestion, nutrients are usually transported throughout the body by diffusion. Cnidarians respire and eliminate the wastes of cellular metabolism by diffusion through their body walls. 20 of 47

20 Response Cnidarians gather information from their environment using specialized sensory cells. 21 of 47

21 Both polyps and medusas have a nerve net, a loosely organized network of nerve cells. Nerve cells 22 of 47

22 Cnidarians also have statocysts, which are groups of sensory cells that help determine the direction of gravity. Ocelli are eyespots made of cells that detect light. 23 of 47

23 Movement Sea anemones have a hydrostatic skeleton. A hydrostatic skeleton consists of a layer of circular muscles and a layer of longitudinal muscles that, with the water in the gastrovascular cavity, enable the cnidarian to move. 24 of 47

24 Reproduction Most cnidarians reproduce both sexually and asexually. Polyps can reproduce asexually by budding. In most cnidarians, sexual reproduction takes place with external fertilization. External fertilization takes place outside the female's body. 25 of 47

25 In the life cycle of Aurelia, a common jellyfish, the female releases eggs into the water, and the male releases sperm. 26 of 47

26 Fertilization occurs in open water. Each zygote grows into a free-swimming larva. 27 of 47

27 The larva eventually attaches to a hard surface and develops into a polyp. 28 of 47

28 The polyp eventually buds and releases young medusas that begin the cycle again. 29 of 47

29 Groups of Cnidarians What are the three groups of cnidarians? 30 of 47

30 Groups of Cnidarians Groups of Cnidarians Cnidarians include: jellyfishes hydras and their relatives sea anemones and corals 31 of 47

31 Groups of Cnidarians Jellyfishes The class Scyphozoa contains the jellyfishes. Jellyfishes live their lives primarily as medusas. The polyp form of jellyfishes is restricted to a small larval stage, and no elaborate colonies ever form. Jellyfishes reproduce sexually. 32 of 47

32 Groups of Cnidarians Hydras and Their Relatives The class Hydrozoa contains hydras and related animals. The polyps of most hydrozoans grow in branching colonies that can extend more than a meter. Within the colony, polyps are specialized to perform different functions. 33 of 47

33 Groups of Cnidarians The most common freshwater hydrozoans are hydras. Hydras differ from other cnidarians in this class because they lack a medusa stage. Instead, they live only as solitary polyps. Hydras reproduce asexually, by budding, or sexually, by producing eggs and sperm in the body wall. 34 of 47

34 Groups of Cnidarians Sea Anemones and Corals The class Anthozoa contains sea anemones and corals, animals that have only the polyp stage in their life cycle. Anthozoans all have a central body surrounded by tentacles. 35 of 47

35 Groups of Cnidarians Many species are colonial, or composed of many individual polyps. Sea anemones are solitary polyps that live at all depths of the ocean. 36 of 47

36 Groups of Cnidarians Most corals are colonial, and their polyps grow together in large numbers. New polyps are produced by budding, and as the colonies grow, they secrete an underlying skeleton of calcium carbonate, or limestone. 37 of 47

37 Ecology of Corals Ecology of Corals Worldwide distribution of corals is determined by a few variables: temperature water depth light intensity 38 of 47

38 Ecology of Corals The corals that build coral reefs require high levels of light these corals rely on algae that capture solar energy, recycle nutrients, and help corals lay down their calcium carbonate skeletons. 39 of 47

39 Ecology of Corals Symbionts provide as much as 60 percent of the energy corals need. This allows coral reefs to live in water that carries few nutrients. 40 of 47

40 Ecology of Corals Many coral reefs suffer from human activity: Silt and sediments from logging, farming, mining, and construction smother corals. Chemical fertilizers, insecticides, and industrial pollutants poison corals. Overfishing upsets ecological balance. Stresses that makes coral reefs susceptible to other threats. 41 of 47