Is the pale string-like structure running the length of the ventral side of the worm the blood vessel?

Yes, the pale string-like structure running along the ventral side of the worm is typically the dorsal vessel, which functions as the worm's blood vessel. This vessel is responsible for circulating hemolymph, the fluid analogous to blood in invertebrates, throughout the worm's body. In some species, this structure may also be referred to as the heart, as it helps pump the hemolymph.