Osmoregulation and Excretion in Platyhelminthes

            Free-living, freshwater flatworms, planarians, have a discrete excretory system (protonephridia), which is primarily osmoregulatory but may have an excretory function as well.  Marine members of the group often lack this system supporting the hypothesis that its main function is osmoregulation.  The system consists of a network of tubules within the animal's body tissues.  One end of the tubule opens through a small pore to the exterior.  The other end of the tube ends blindly within the body in a spherical structure containing long cilia - these are called flame cells (Figure 10).  Excess water (and possibly wastes) enters the flame cell system and is propelled through the tubules toward the outside by the beating of the cilia (the "flame").

Figure 10: Planarian Excretory System (top) and One Flame Cell (bottom).

                    (Note the long cilia ("flame") which move material toward the excretory canal)

            Unfortunately, the flame cell system of planarians are difficult to locate, even in live animals and will not be observed during this lab.

Respiration and Circulation in Platyhelminthes

            Platyhelminthes lack respiratory and circulatory systems.  These animals are small and are always found in a moist environment whether they are free-living or parasitic. When present (as in Dugesia sp.), the gastrovascular  system (meaning gut-transport)  serves to transport nutrient materials.  Otherwise, all internal transport occurs by simple diffusion through and between cells of the small body.

Reproduction in Platyhelminthes

            Planarians are hermaphroditic organisms that have both male and female sex organs in the same individual.  Copulation in flatworms is usually mutual, each partner inseminating the other.  The reproductive system of planaria is difficult to locate, even in live animals, and will not be observed during this lab.

Note that the body plan, digestive systems, and methods of reproduction of tapeworms and liver flukes are quite different compared to planarians.