BIO 5 PLANT BIOLOGY

PHYLUM CNIDARIA

 

1.                  The Phylum Cnidaria includes a variety of animals: Hydra, Portuguese man-o-war, jellyfish, sea anemones, corals, and sea fans. All of the animals in the Phylum are aquatic. Most are marine, but a few, like Hydra live in freshwater.

2.                  Like the sponges (Phylum Porifera), animals in the Phylum Cnidaria are multicellular. However, unlike the sponges in which the cells are independent of one another, the cells of animals in the Cnidaria are organized into tissues. Tissues are groups of closely associated similar cells that carry out carry out a specific function. Examples of tissues are muscle tissue or nerve tissue.

3.                  The animals in the Phylum Cnidaria have radial symmetry. (To determine symmetry, the animal has to be viewed from the top, rather than from the side like the Hydra in the diagram used in lecture.)

4.                  Animals in the Phylum Cnidaria also have a cul-de-sac gut or gastrovascular cavity (gvc). This means that, while the animal has a digestive cavity that allows it to ingest relatively large prey, there is only one opening to the gvc.

5.                  The food eaten by Hydra and other animals in the Phylum Cnidaria is other animals. Animals that eat other animals are called carnivores. All animals in the Cnidaria have specialized cells called cnidocytes (stinging cells). These cells release an elongated portion called the thread which injects a paralyzing toxin into prey animals. The cnidocytes are most abundant on the tentacles of the animal. In Hydra, the tentacles are muscular and assist in feeding by placing the paralyzed animal into the mouth. Once the prey has been placed into the mouth, it enters the gvc.

6.                  Despite the fact that Hydra is a predatory carnivore, the animal is sedentary. This means that, while capable of movement, the animal moves infrequently. When the animal does move it either glides slowly on its base or summersaults. For all practical purposes, the prey animals the Hydra eats must accidentally brush it and trigger the cnidocytes for the prey to be captured by Hydra.

7.         Just as Hydra is not an active carnivorous animal, it also senses and responds to it's environment only minimally. A network of nerve cells (called a nerve net) crisscrosses the animal. The nerve net is connected to sensory cells located among the other cells of the animal. Hydra is attracted to food, moderate light and moderate temperatures. It avoids light and temperature extremes and harsh chemicals. It also responds to touch.

8.         Once food has been senses, paralyzed by the cnidocytes and ingested into the gvc, digestion of the prey animal begins. This is a two step process. The first step is called extracellular digestion. It takes place in the gvc. The gastrovacular cavity is lined with a tissue called the gastrodermis. These cells release digestive enzymes into the gvc. The enzymes break the animal down into food particles. The second step in food digestion is called intracellular digestion. This occurs within the gastrodermis cells. Food particles enter these cells by endocytosis, and food vacuoles are formed. Lysosomes release digestive enzymes into the food vacuoles, and the food particles are broken down into food molecules: sugars, lipids, protein and/or amino acids. These food molecules then enter the cytoplasm of the cell by diffusion, facilitated diffusion, or active transport. The food molecules then are used by the gastrodermis cell or diffuse into the other tissue of the Hydra, the epidermis. All cells of the Hydra use food molecules in aerobic respiration or to build cell structures. The two tissue layers of the Hydra, the gastrodermis and epidermis are separated from each other by a jelly-like material called the mesoglea.

9.         The processes of digestion and aerobic respiration produce waste materials that must be eliminated from animals. Food vacuoles release digestive waste into the gvc, and this digestive waste joins other digestive waste present in the gvc from extracellular digestion. This waste material leaves the gvc through the mouth. The animal contracts (gets shorter and wider) and the waste shoots out of the mouth. Nitrogen waste (ammonia) is produced as a result of protein digestion. The ammonia diffuses from the Hydra into the water surrounding the animal. Carbon dioxide waste is produced as a result of aerobic respiration. This too leaves the animal by diffusion.

10.       Oxygen is needed by animals to carry out aerobic respiration. This enters the cells of Hydra by diffusion from the water surrounding the animal into the cells of the animal.

11.            Members of the Cnidaria have several special features. These include the cnidocytes that are found in no other types of animals and are unique to the Phylum. A second feature is the ability of these animals to undergo budding. Mitosis produces buds (new individuals) in Hydra, for example. The bud eventually becomes detached from the parent and then lives an independent existence. Another special feature is the alternation of body forms that is present in many species of the Phylum Cnidaria. Jellyfish (Class Scyphozoa) is a good example of the alternation of body forms. Adult jellyfish are the medusa body form. This body forms swims by contractions of the body. The medusa produces either sperm cells or eggs. The gametes are formed by meiosis, are 1N (haploid) and are released into the ocean. Sperm and egg cells fuse (fertilization) and a zygote is produced. The zygote undergoes mitosis to form swimming immature stages. The immature stage eventually settles on the ocean floor or a structure like a reef and a sedentary polyp stage is produced. The polyp undergoes budding, and the buds are small swimming medusae. These then grow to adult size.

12.       The Phylum Cnidaria has an impact on people in two ways. Included within the phylum are the corals. These form massive reefs in tropical oceans. Coral reefs are complex and rich ecosystems that provide rich fisheries for both fin fish and shellfish eaten by humans. The second way that the cnidarians have an impact on people is that the stinging cell toxins are, minimally, irritants and can be deadly to people. If a person brushes against fire coral, the person's skin feels as if it were on fire. If a person is stung by the sea wasp jellyfish death usually follows within minutes.