The dorsal surface has a dark median mid dorsal line due to the dorsal blood vessel along the longitudinal axis of the body

The correct answer is option 'B' - Dorsal blood vessel.

Explanation:
The earthworm is a segmented organism and belongs to the phylum Annelida. It has a well-developed circulatory system that helps in the transportation of nutrients, gases, and waste products throughout the body. The circulatory system of an earthworm consists of blood vessels and a pumping organ called the dorsal blood vessel.

The dorsal blood vessel is a contractile tube located along the midline on the dorsal side (upper side) of the earthworm's body. It runs from the anterior (front) to the posterior (rear) end of the body, just above the digestive system. The dorsal blood vessel is the main circulatory organ responsible for pumping blood throughout the body.

The dorsal blood vessel plays a crucial role in the earthworm's circulatory system. It helps in maintaining the flow of blood and nutrients to all the body segments. It receives blood from smaller vessels called hearts located in each segment of the earthworm's body.

The blood in the earthworm's circulatory system is colorless and contains hemoglobin, which helps in the transport of oxygen. The dorsal blood vessel contracts rhythmically, creating a pulsating flow of blood throughout the body. This rhythmic contraction of the dorsal blood vessel ensures the circulation of oxygen, nutrients, and waste products.

In addition to the dorsal blood vessel, earthworms also have other blood vessels, including ventral blood vessels, lateral blood vessels, and subneural blood vessels. These vessels help in the distribution of blood to different parts of the body.

In conclusion, the dorsal median line in an earthworm is due to the presence of the dorsal blood vessel. This vessel is responsible for the circulation of blood and nutrients throughout the earthworm's body.