Comparative Anatomy Lab 1: Cnidarians The Cnidarians are an ancient assemblage of organisms whose ancestry can be traced back more than 700 million years. This marks them as one of the earliest stock of metazoan fauna. Cnidarians are wholly aquatic and mostly marine. The phylum includes jellyfish, sea anemones, corals, hydra, siphonophores (e.g. Physalia - Portugese Man O War) and box jellyfish. These organisms can be found in the open ocean as well as in benthic environments. Some species occur at considerable depth. All Cnidarians have a soft, radially symmetrical body that might seem like an easy source of food. However, Cnidarians have relatively few natural predators. This is largely due to the protection provided by their stinging nematocysts. Cnidarians have a distinct internal digestive cavity (enteron) separate from the primary body cavity (mesoglea). They are also diploplastic having distinct ectoderm and endoderm tissue layers. Finally, Cnidarians display bimorphism, a sessile polyp form and a freeswimming or free floating medusal form. During this laboratory you will survey a number of artifacts, pictures, and specimens that will introduce you to the diversity of Cnidarians in the world s oceans. As you examine these objects, use this form to record your observations and collect information about this amazing group of animals. You will be given a companion guide which includes much useful information about Cnidarians. Use this to verify your knowledge about Cnidarian anatomy and biology. Checkpoint: Highlight or underline five major characteristics in the above paragraphs that distinguish the Cnidarians. Put a star next to the two characteristics that are exclusive to Cnidarians. In this dissection you will compare a jellyfish (Aurelia sp.) and a sea anemone (Metridium sp.) Follow the directions below to examine and collect notes on these organisms.
Dissection 1: Jellyfish (Aurelia sp.) Position your jellyfish so that the aboral or top of the jellyfish is facing upward. Sketch the jellyfish in the box below. Label the following parts: bell or float and tentacles. Note the diameter of your jellyfish on your drawing. Aurelia sp. aboral view Describe the texture and appearance of jellyfish tissue: Turn your jellyfish over and examine the oral side (bottom) of the organism. The medusa of Aurelia has short tentacles, a flat mouth/anus opening, and four elongated oral arms that surround the mouth. These jellyfish are common along Atlantic and Pacific coastlines of the United States. This jellyfish can reach up to 15 cm in length. Instead of using long tentacles to trap prey, these jellyfish are suspension feeders. Drifting plankton stick to a mucus layer on the underside of the bell of the jellyfish. The oral arms scrape this plankton-filled mucus into the mouth. The gonads or reproductive organs of this jellyfish are also visible in the bell. They are usually a pinkish-orange coloration and occur in four loops with the mouth at the center.
Use the dissecting tools to make an incision in the digestive tissue of the jellyfish. Observe the differences in the gastrodermis (digestive tissue) and the epidermis (tissue of the bell and tentacles). Label the tentacles, gonads, oral arms, and the mouth/ anus Aurelia sp. oral view What is the difference between an oral arm and a tentacle? Trace the digestive tract of the jellyfish. How might food move through the organism? How might eggs and sperm be released from the gonads?
Dissection 2: Sea anemone (Metridium sp.) Position your sea anemone so that the oral side is upward. Sketch the external anatomy of the sea anemone in the box below. Label the following parts: body, tentacles, mouth, oral disc, and pedal disc. Use the figure on the next page to assist you. Metridium sp. external anatomy Note the height and width of your anemone on your drawing. Describe the texture and appearance of jellyfish tissue: Lay your sea anemone on its side. Starting at the mouth, make an incision from the oral disc through the side of the anemone down to the pedal disc. Use the dissecting pins to pin open the sea anemone so that you can observe its internal anatomy. Sketch the internal anatomy of the sea anemone below. Label the mouth, gastrodermis, digestive mesenteries, and any gonads (if visible). Use the figure on the next page to assist you.
Metridium sp. internal anatomy! Compare and contrast the tissue, body organization, and anatomy of the jellyfish and the sea anemone.
Based on their anatomy alone, how is the niche of each of these organisms different?