The animal Phylum Cnidaria is made up of animals with a primitive level of organization: they have only two layers of cells, an outer, somewhat skin-like ectoderm and an inner digestive lining, endoderm or gastroderm, and a layer of jellyish material loosely occupied with replacement cells in between. A particular feature of this group are their cnidocytes, stinging cells containing poisoned harpoons that are used for catching prey and defense. Many members of the Cnidarians live attached to the ocean floor - such as corals and sea anenomes - and some, such as jellyfish, swim weakly in the open water. In this lab, we will look at a few examples from this group.


Cnidarians. Living Hydra.

Hydras are common freshwater animals - you can find them during warm weather if you know what to look for. They are one of the few successful fresh water Cnidarians, which makes the easy to keep in the lab; however, they're not very typical Cnidarians. Hydras have a permanent polyp body form, which in most Cnidarians is one of two alternating body forms, but Hydras lack the other, medusa stage, a weak-swimming stage that would not do well in moving inland waters.

Lab Instructions. Get a living hydra on a slide, but do not put a cover slip on it, (yet). Remember, without a cover slip you're limited to just low and middle power. Often a hydra will contract when transferred; be patient and it should relax again.

1. In the space below, draw your Hydra, and label the following structures: the BASE, a disk on the other end of the BODY from the TENTACLES; in the mound where the tentacles meet the body, the MOUTH; the DIGESTIVE CAVITY should show through the body as a shadow.

2. Describe how the Hydra moves once it's relaxed and attached.

3. Describe how the Hydra reacts when startled - it may have been when you picked it up, or it may react to your tapping the slide.

4. Hydras have a variety of reproductive structures, both sexual (spermaries, slightly pointed bumps, eggs or ovaries, broad low mounds, or embryos, like attached balls) and asexual (buds, like miniature Hydras attached to the body). If your Hydra has such a structure, describe it here.

5. We're going to try to get the Hydra to feed. Get one to a few Daphnia, small crustaceans, added to your slide. Try to limit the water drop size around the animals, but leave enough for the Daphnia to swim in and do not let the drop dry out! Observe and record here how the animals react to each other. Give the plenty of chances - this sometimes takes a while.

6. Return the Hydra to the bottle and pick up a stained, preserved specimen. At high power, find and draw a small part of a tentacle. Label the egg-shaped trigger chambers, the CNIDOCYTES (often there will be harpoons visible inside), and the mound-like CNIDOBLASTS around them.

Cnidarians - Obelia Hydroid Colony.

Polyp Form.

Obelia is a small marine Cnidarian that lives in colonies, with several animals connected by stalks or stems. There are two basic forms in the colony members: feeding and defending HYDRANTH polyps, with spindly TENTACLES around a hidden mouth; vase-shaped GONANGIUM members give rise asexually to the medusa stage of the Obelia, which are visible in many gonangia as small, disklike MEDUSA BUDS (some gonangia may be empty, having released their medusae through their GONOPORE openings, which sometimes have plugs in them) The entire colony is surrounded by a covering, the PERISARC, which is called a HYDROTHECA around the hydranths and a GONOTHECA around the gonangium.

On the microscope slide, find an area with both hydranths and full gonangia - the best area is toward the middle of the colony. Draw enough of the colony to include all of the structures, and label those structures.


Cnidarians - Obelia Medusa Stage.

Get an Obelia medusa slide - there are two small specimens on each slide. You want a specimen that looks round. These are medusae, tiny jellyfish (larger jellyfish may or may not be a medusa stage). Medusae reproduce sexually, which the immobile polyps can't easily do, producing a planula larva which will settle to the bottom and, if conditions are favorable, become a new polyp colony. Draw and label a medusa - it is thick enough that it may not be in focus all at once, and some structures may be setting on top of others. A medusa is like an upside-down version of a polyp - the TENTACLES point down, attached to the edge of the bell with TENTACULAR BULBS. The MOUTH is also underneath, leading into a MANUBRIUM chamber that connects to the central STOMACH. A series of canals help distribute materials - four RADIAL CANALS connect the stomach to the RING CANAL around the edge of the bell. Each radial canal travels past a GONAD. Each medusa is male or female, but identifying which is which is very difficult.

Cnidarians - Aurelia Planula Stage.

The planula is a larval stage in the life cycle of many bottom-dwelling Cnidarian species. Sperm and egg cells from medusae form a zygote, which grows into a planula. The planula floats in the currents for a while, then sinks to the bottom and crawls around looking for a good spot to start a new hydroid colony.

On the slide, there is usually just one specimen, barely visible to the naked eye, often near the edge of the cover slip. They are stained red. Draw the larva. It's a multicellular organism with an outer layer covered with cilia, but the cilia are usually too tiny to make out. There really are no recognizable structures to label.

A common "trick" of evolution is to produce mature versions of larval forms, which can quickly evolve into dramatically different organisms. The simplest members of the Platyhelminthes, flatworms such as we'll be looking at in the next lab, bear a striking resemblance to these planula larvae.


First Written 1990;  Last Update 2001;  Web Version 2001,  M. McDarby




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