sacs of specialized excretory cell found in the simplest freshwater invertebrates such as Turbellaria

The sacs of specialized excretory cells found in the simplest freshwater invertebrates such as Turbellaria are known as Protonephridia.

Here are the steps of how they function:

1. Protonephridia are primarily involved in osmoregulation, which is the process of maintaining the balance of water and salts within the body of the organism.

2. They are composed of a network of dead-end tubules connected to external openings, called nephridiopores.

3. The tubules are lined with ciliated cells which help in the movement of waste materials.

4. The waste materials are collected from the body fluid by the flame cells (also known as solenocytes), which are located at the end of the tubules.

5. The flame cells have a cluster of cilia that create a current, pushing the waste materials through the tubules.

6. The waste materials are then excreted out of the body through the nephridiopores.

7. This system helps the organism to maintain its internal balance of water and salts, which is crucial for its survival in freshwater environments.