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sp02u2p3 - Unit Two MBNMS Jellies Lesson Objectives...

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Unit Two MBNMS ©Project Oceanography Spring 2002 56 Jellies Lesson Objectives: Students will be able to do the following: Distinguish between a polyp and a medusa stage Characterize locomotion and feeding within the true jellies Describe a Cubozoan Key Concepts: Cnidarians, classification, medusa, polyp, mesoglea, nematocysts Cnidarians Jellies with their umbrella shaped bells and ribbon like tentacles can be seen drifting along in the ocean currents. These fascinating creatures belong to a much larger group of animals known as Cnidarians . These “jellies” are all soft bodied, aquatic animals that share some distinctive characteristics. These animals have sac-like bodies with a hollow central cavity. The body is made up of a jelly-like substance called mesoglea . This mesoglea is 98% water and is found between two cellular layers. The only body opening is a mouth. Tentacles that contain stinging cells surround the mouth. These stinging cells or nematocysts have a variety of functions depending on the type of cell. Some are used as “sticky” cells to tangle or glue their prey, others release poisons that stun and kill prey. Cnidarians can be found with two different body shapes or plans: polyps and medusae . An anemone is an example of a jelly with a typical polyp shaped body. Its body looks like a cylinder. There is a mouth at one end with a muscular foot or pedal disc at the other end. These animals are often sessile staying attached to a substrate. A jelly with its bell shaped body and long tentacles is an example of a medusa. The medusae are seen swimming or drifting freely in the water. Cnidarians can be further divided into groups called classes. The four classes of Cnidarians presently recognized by scientists are the Anthozoans, the Hydrozoans, the Scyphozoans, and the Cubozoans. Anthozoans are the largest class of jellies. This class includes corals and sea anemones. These animals only have polyp stages. Hydrozoans have the most variety within their class. They include hydroids, siphonophores, fire corals, and other medusae. Hydroids come in the polyp form (often looking like a plant) or the medusa form. Siphonophores include colonial animals such as the
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Unit Two MBNMS ©Project Oceanography Spring 2002 57 Portuguese Man-O-War. This animal looks like a true jelly with its transparent float and long tentacles, but it is really many animals living together in a colony. Hydroids can be mistaken for plants and have a complex life cycle with animals in this class alternating between the polyp and medusa stages. Scyphozoans are the true jellies. Some of these animals have only medusa stages while others alternate between medusa and polyp. Animals in this class swim by contracting their bell and expelling water from the underside. The Cubozoans are the most recently designated class of Cnidarians. They were originally classified as an Order within the Class Scyphozoa. Cubozoans are the box jellies. They are identified by their box shaped bells. The jellies
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