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finger sponge large sponge Scypha SWA og her with ores e w word hollow long tures with fully pours - Phylum Cnidaria Cnidarians are the simplest animals with definite tissues. They are diploblastic, which means they have et arranged in two body layers, the outer epidermis layer that arises from an ectoderm in the embryo and the gastrodermis layer lining the gastrovascular cavity that arises from the endodermal layer in the embryo. As in sponges, a jellylike substance, the mesoglea, lies between the two layers. This mesogea is not a tissue, there is no mesodermal tissue in the embryos of cnidarians. The name "enidarian" comes from endocytes". the cells containing nematocysts. The group contains the classes Anthozoa (sea anemones and corals). Cubozoa box jellies). Hydrozoa (hydroids), and Scyphozoa (jellyfishes). There are two basic body forms for the cnidarians. In both cases the symmetry is radial. The polyp stage is shaped like a vase. The opening to the gastrovascular cavity is at the top, or oral, end of the organism. The lower region of the cavity is closed making the mouth a two way opening-food comes in through the mouth and wastes go out through the mouth. The aboral end of the organism is the bottom of the organism, and is attached to a substrate. The medusa stage is the second type of body form in cnidarians, it is shaped like a bell, with the opening to the gastrovascular cavity at the "bottom", but still oral, ond of the organism and the closed region, again, the aboral end, is at the top of the organism, Medusae are free-swimming organisms and so are not attached to a substrate. In most groups, these two body forms alternate in the organism's de cycle. Some groups have lost one or the other of the two body forms and exist only as polyps or only as medusae. Two other basic characteristics of the group are the tentacles that surround the mouth, and a diffuse nerve net that provides a small amount of nervous coordination. • We will first study the polyp Hydra. To which class of Cnidariams does Hydra belong? da zoon • Fill in the correct section in the Phylum Cnidaria chart with the information pertaining to Hydra Obtain a living specimen of Hydra, place it in a Petri dish and fill the dish at least hallway with water as indicated by your instructor. (Do not use tap water, it contains traces of chemicals toxic to the Hydra. Observe the Hydra using a dissecting microscope. Draw a sketch of the organism and label the following structures: tentacles, cnidocytes, mouth, and gastrovascular cavity. Notice the behavior of the Hydra. After watching the activity of the animal for several minutes, very gently touch the tip of its tentacles with a dissecting needle. What is the reaction? • Very gently touch the side of the body with the dissecting needle. What is the reaction?

Answer 1

Answer 1

Hydra have a number of stinging cells on their tentacles called nematocytes. If the ends of the tentacles are touched with a dissecting needle, tentacles trigger nematocyst discharge.

Answer 2

When we touch the body of hydra with a dissecting needle, the body contracts.

As described above, Hydras have a classic nerve net, i.e., a network of neurons, joining the whole body of the organism together. In hydra, the epidermis also consists of a nerve network. This nerve network is made up of neurons with many axons and these cells are called ganglion cells. These cells are especially numerous at the end of tentacles. Along with the nerve net, the epidermal cells also has a neuroid system, made up of gap junctions. The cell membrane of the epidermal cells acts as capacitors. when there is a mechanical touch, there is a flow of electric current across the cell membrane, thus passing the electric signals to the neuroid and the nervous system