Blood Vessels in Pheretima with Valves - Dorsal

Explanation:
Pheretima, commonly known as earthworms, are segmented invertebrates that belong to the phylum Annelida. The circulatory system of Pheretima consists of blood vessels that transport blood and nutrients throughout the body. The blood vessels are divided into three main types: dorsal, ventral, and lateral oesophageal vessels.

Valves are structures that prevent the backflow of blood and maintain the unidirectional flow of blood in the circulatory system. In Pheretima, these valves are present in the dorsal vessels.

The dorsal vessels are located above the digestive tract and run along the length of the body. They are responsible for carrying blood from the anterior to the posterior end of the body. The valves in the dorsal vessels prevent the backflow of blood and ensure that the blood flows in one direction.

In contrast, the ventral vessels are located below the digestive tract and carry blood from the posterior to the anterior end of the body. However, they do not have valves, and the blood flow can reverse in case of any disturbance.

Similarly, the lateral oesophageal vessels are located on either side of the oesophagus and connect the dorsal and ventral vessels. Although they do not have valves, they play an important role in regulating the blood flow and pressure in the circulatory system.

In conclusion, the correct answer is option 'A' - Dorsal blood vessels in Pheretima have valves that prevent the backflow of blood and maintain the unidirectional flow of blood in the circulatory system.