Protonephridia

Platyhelminths, such as Planaria, have a network of fine tubules running throughout the body. The tubules are connected to bulb-like structures called flame cells. These cells contain cilia which move the fluids toward the excretory pores located throughout the epidermis. The cells are called flame cells because when you observe a squash under the microscope, the beating cilia look like a flickering candle.

The fluids that diffuse into the excretory system contain water and many dissolved materials. Presumably materials that the animal needs are reclaimed. The primary function of the flame cells is to regulate water. Excretion is secondary because most of the metabolic wastes diffuse into the gut and are eliminated through the mouth.

• What does protonephridia mean?
• If the flame cells primarily regulate water, would the fluid leaving this body be hyperosmotic or hyposmotic to the planarian body fluids?
• Why do Planaria need flame cells when they are so small and flat? Can't diffusion be used to eliminate wastes and excess water?

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