Student Resources

Platyhelminthes -Workbook Questions

Describe the cephalization of the flatworm’s nervous and sensory systems.  (p. 32) 

Bilateral symmetry and directed movement result in animals that have an anterior end that senses where the animal is going. In flatworms, the nervous system consists of two main nerve cords that run down the sides of the animal. Each of the two cords is connected to the other by lateral nerves, and the result is the “ladder-like” organization of the nervous system characteristic of flatworms. At the anterior end are two pigmented eye cups, eye spots, that detect light, and the cheek-like auricles that are chemosensory. The presence of these sensory structures is associated with an increase in nerves and nerve tissue to interpret the information. This is the beginning of a brain—another important part of cephalization. 

How does a flame cell work?  (p. 32) 

Flame cells, also called protonephridia, have a cup-shaped cap, or terminal cell, that connects with a hollow cellular tubule that opens to the outside through the nephridiopore. The cell walls on the side of the cap cell are thin, and cilia extend from the inside near the tip of the cap cell. Biologists’ best guess to how this works is that the cilia inside the cup-shaped cell beat propelling fluid inside away from the cap cell, down the tubule, and out the nephridiopore. Movement of fluid away from the cup creates a negative pressure, suction, and this pulls fluid outside the wall of the flame cell inside across the thin membranous part of the cap cell. Like all membranes, only water and small molecules dissolved in it can pass through the membrane, and as water enters the flame cell it brings with it any dissolved nitrogenous wastes. In higher animals, osmoregulation and excretion are closely tied to each other, but in flatworms the two are still separated. Flatworms loose most of their nitrogenous wastes by diffusion across the body surface. Protonephridia are more important for osmoregulation, especially in freshwater forms, where it’s important to remove excess water that diffuses into the body. 

What advantage could there be to being a monoecious organism?  (p. 33) 

Even though monoecious organisms have both male and female reproductive systems, they rarely fertilize their own eggs. Either the two systems mature at different times or the events of sperm transfer and fertilization are separated. They do have one advantage that dioecious organisms do not have. When dioecious organisms mate, only one parent produces new offspring. That’s not the case with monoecious animals. When monoecious meet, they both can receive sperm from their partners and will later fertilize their own eggs with the sperm they received when they mated. The result is that when monoecious animals mate, both parents produce young. Monoecious organisms are often sessile or slow moving, and there is an advantage to having both animals produce young when there is a chance encounter. 

What are the different stages in the fluke life cycle, and what does each contribute to the cycle?  (p. 34) 

To survive, a parasite must move between its different hosts, and each of the stages in the life cycle is involved either in increasing the numbers of the parasite inside the host or it is modified for moving between hosts. Fertilized eggs are deposited into the environment and hatch to form miracidia that swim to and penetrate the snail, the first intermediate host. Once inside the snail, each miracidium changes into a sporocyst, which undergoes asexual larval amplification until each of the sporocysts is filled with numerous redia. The redia change into motile cercaria, released into the water. Depending on the fluke species, cercaria may invade a second intermediate host to form dormant metacercarial cysts or penetrate the final definitive host. In species that have metacercaria, the definitive host is infected by consuming meat that contains the cysts. Once inside the definitive host, adults form and produce huge numbers of fertilized eggs that will become miracidia to complete the life cycle. 

How can you tell if your cross section is anterior to, posterior to, or through the pharyngeal region?  (p. 34) 

Dugesia, or Planaria, is a triclad fluke, meaning that its digestive tract is divided into three main branches, one anterior and two posterior to the pharyngeal opening. That’s your clue to where your section is from. If it’s anterior to the pharynx, you won’t see any sign of the pharynx in the center of the section, only the opening of the anterior branch of the digestive tract. Be careful, though. There are diverticula, small branches, that extend from this anterior branch, and they may also be visible in your slide. If your section is through the pharynx, you’ll see it and its musculature along with the pharyngeal cavity that surrounds it. If the section is posterior to the pharynx, you’ll see the paired large opening of the alimentary tract with no sign of the pharynx. Again, be careful that you don’t confuse the diverticula of the posterior branches with the main alimentary cavity. 

How do tapeworms and flukes protect themselves from being digested or attacked by their host?  (p. 36) 

The outside of most animals is covered with a cell layer, the epidermis, which sits on top of a basement membrane. If the epidermis is damaged, the nuclei of the damaged cells can no longer produce the required messages and materials to repair the cell. The only way to reverse the damage is by replacing damaged cells with new cells created by mitosis. Tapeworms and flukes are covered with a tegument consisting of cells arranged in a syncytium, with no cell boundaries separating one cell from the other. From the syncytial covering, cytoplasmic extensions connect the outer cell surface to the cell bodies located below the basement membrane and musculature. The cell body is protected from any damaging attacks from the host, and the nucleus can still control the repair, synthesis, and replacement of damaged tegument above. There is also evidence that the nucleus may synthesize inhibitors to hydrolytic enzymes that attack the surface of the animal.

Protozoa || Porifera || Cnidaria || Platyhelminthes || Nematoda || Annelida || Mollusca || Arthropoda
Echinodermata || Chordate Origins || Jawed Fishes || Amphibia || Mammalia